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
 
a782ab4c82'>packages</a>/<a href='/guix/log/gnu/packages/simh.scm?id=308877be17b2c2c92d972e105e8cc6a782ab4c82'>simh.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/packages/simh.scm?id=308877be17b2c2c92d972e105e8cc6a782ab4c82&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-08-31 10:42:23 +0200'>2024-08-31</span></td><td><a href='/guix/commit/gnu/packages/simh.scm?id=8383b9f04abcc1a5772a9bf8fae49b8c7794d752'>gnu: simh: Import the correct set of modules.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/simh.scm (simh) [arguments]
&lt;modules&gt;: Replace %default-gnu-imported-modules with %default-gnu-modules.

Change-Id: I11a5cd3619ada3a280e990741b513f6c20a1713a
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2024-08-31 10:42:16 +0200'>2024-08-31</span></td><td><a href='/guix/commit/gnu/packages/simh.scm?id=f59df1aa3c104aabff3291f6b8b155fb01aebc06'>build-systems: gnu: Export %default-gnu-imported-modules and %default-gnu-mod...</a><span class='msg-avail'>...<span class='msg-tooltip'>Until now users would have to cargo cult or inspect the private
%default-modules variable of (guix build-systems gnu) to discover which
modules to include when extending the used modules via the #:modules argument.

The renaming was automated via the command:

  $ git grep -l %gnu-build-system-modules
    | xargs sed 's/%gnu-build-system-modules/%default-gnu-imported-modules/' -i

* guix/build-system/gnu.scm (%gnu-build-system-modules): Rename to...
(%default-gnu-imported-modules): ... this.
(%default-modules): Rename to...
(%default-gnu-modules): ... this.  Export.
(dist-package, gnu-build, gnu-cross-build): Adjust accordingly.

Change-Id: Idef307fff13cb76f3182d782b26e1cd3a5c757ee
</span></span></td><td>Maxim Cournoyer</td></tr>