Fix build failure with GCC 10.

Taken from upstream:
https://github.com/dcnieho/FreeGLUT/commit/b9998bbc1e1c329f6bf69c24606a2be7a4973b8c

diff --git a/src/fg_gl2.c b/src/fg_gl2.c
--- a/src/fg_gl2.c
+++ b/src/fg_gl2.c
@@ -27,6 +27,20 @@
 #include "fg_internal.h"
 #include "fg_gl2.h"
 
+#ifndef GL_ES_VERSION_2_0
+/* GLES2 has the corresponding entry points built-in, and these fgh-prefixed
+ * names are defined in fg_gl2.h header to reference them, for any other case,
+ * define them as function pointers here.
+ */
+FGH_PFNGLGENBUFFERSPROC fghGenBuffers;
+FGH_PFNGLDELETEBUFFERSPROC fghDeleteBuffers;
+FGH_PFNGLBINDBUFFERPROC fghBindBuffer;
+FGH_PFNGLBUFFERDATAPROC fghBufferData;
+FGH_PFNGLENABLEVERTEXATTRIBARRAYPROC fghEnableVertexAttribArray;
+FGH_PFNGLDISABLEVERTEXATTRIBARRAYPROC fghDisableVertexAttribArray;
+FGH_PFNGLVERTEXATTRIBPOINTERPROC fghVertexAttribPointer;
+#endif
+
 void FGAPIENTRY glutSetVertexAttribCoord3(GLint attrib) {
   if (fgStructure.CurrentWindow != NULL)
     fgStructure.CurrentWindow->Window.attribute_v_coord = attrib;
diff --git a/src/fg_gl2.h b/src/fg_gl2.h
--- a/src/fg_gl2.h
+++ b/src/fg_gl2.h
@@ -67,13 +67,13 @@ typedef void (APIENTRY *FGH_PFNGLENABLEVERTEXATTRIBARRAYPROC) (GLuint index);
 typedef void (APIENTRY *FGH_PFNGLDISABLEVERTEXATTRIBARRAYPROC) (GLuint);
 typedef void (APIENTRY *FGH_PFNGLVERTEXATTRIBPOINTERPROC) (GLuint index, GLint size, GLenum type, GLboolean normalized, GLsizei stride, const GLvoid *pointer);
 
-FGH_PFNGLGENBUFFERSPROC fghGenBuffers;
-FGH_PFNGLDELETEBUFFERSPROC fghDeleteBuffers;
-FGH_PFNGLBINDBUFFERPROC fghBindBuffer;
-FGH_PFNGLBUFFERDATAPROC fghBufferData;
-FGH_PFNGLENABLEVERTEXATTRIBARRAYPROC fghEnableVertexAttribArray;
-FGH_PFNGLDISABLEVERTEXATTRIBARRAYPROC fghDisableVertexAttribArray;
-FGH_PFNGLVERTEXATTRIBPOINTERPROC fghVertexAttribPointer;
+extern FGH_PFNGLGENBUFFERSPROC fghGenBuffers;
+extern FGH_PFNGLDELETEBUFFERSPROC fghDeleteBuffers;
+extern FGH_PFNGLBINDBUFFERPROC fghBindBuffer;
+extern FGH_PFNGLBUFFERDATAPROC fghBufferData;
+extern FGH_PFNGLENABLEVERTEXATTRIBARRAYPROC fghEnableVertexAttribArray;
+extern FGH_PFNGLDISABLEVERTEXATTRIBARRAYPROC fghDisableVertexAttribArray;
+extern FGH_PFNGLVERTEXATTRIBPOINTERPROC fghVertexAttribPointer;
 
 #    endif
 
</a>/<a href='/guix/log/gnu/services?id=685b4f8d6f4cd287269d80a0365e26d4f7307f55'>services</a>/<a href='/guix/log/gnu/services/authentication.scm?id=685b4f8d6f4cd287269d80a0365e26d4f7307f55'>authentication.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/authentication.scm?id=685b4f8d6f4cd287269d80a0365e26d4f7307f55&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-04-24 22:21:35 +0200'>2019-04-24</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=9374cbd1fb29395a5e849e95e8a249e5f9f944c0'>services: fprintd: Provide polkit policy.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (fprintd-service-type)[extensions]: Add
polkit-service-type.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-04-21 10:40:19 +0200'>2019-04-21</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=dc01978288318a8155e2e2657be9b278a754ebac'>services: fprintd: Use define-configuration.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm: Use define-configuration in fprintd.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-04-17 08:51:11 +0200'>2019-04-17</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=0682f084635cdc289aabafc4b2583639e00a28b3'>services: fprintd: Fix service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (&lt;fprintd-configuration&gt;)[ntp]: Rename to...
[fprintd]: ...this.
(fprintd-dbus-service): New procedure.
(fprintd-service-type): Use it.
[default-value]: Add value.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2019-03-20 20:31:15 +0100'>2019-03-20</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=c16423f143919916a5273761d7ed29bd49f14519'>services: Add nslcd-service-type.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm (nslcd-service-type, nslcd-configuration,
%nslcd-accounts): New variables.
(uglify-field-name, value-&gt;string, serialize-field, serialize-list,
ssl-option?, tls-reqcert-option?, deref-option?,
comma-separated-list-of-strings?, serialize-ignore-users-option, log-option?,
serialize-log-option, valid-map?, scope-option?, serialize-scope-option,
map-entry?, list-of-map-entries?, filter-entry?, list-of-filter-entries?,
serialize-filter-entry, serialize-list-of-filter-entries, serialize-map-entry,
serialize-list-of-map-entries, nslcd-config-file, nslcd-etc-service,
nslcd-shepherd-service, pam-ldap-pam-services, pam-ldap-pam-service,
generate-nslcd-documentation): New procedures.
* gnu/tests/ldap.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (LDAP Services): Document it.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-06-23 00:21:23 +0200'>2018-06-23</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=812f6bd82dd584cc0551a687ec52315ae8ee4afb'>services: Fix "authentication" service name.</a><span class='msg-avail'>...<span class='msg-tooltip'>Follow-up to 7f93bbd5aadf0427190769fba8f478c29e37b4f4.

* gnu/services/authentication.scm: Fix module name.
</span></span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2018-06-23 00:14:27 +0200'>2018-06-23</span></td><td><a href='/guix/commit/gnu/services/authentication.scm?id=7f93bbd5aadf0427190769fba8f478c29e37b4f4'>services: Add fingerprint identification service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/authentication.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Miscellaneous Services): Document it.
</span></span></td><td>Danny Milosavljevic</td></tr>