Building p7zip with GCC 11 or newer normally fails with error messages like

  ../../../../CPP/7zip/Archive/Wim/WimHandler.cpp:308:11: error: use of an
  operand of type ‘bool’ in ‘operator++’ is forbidden in C++17
    308 |           numMethods++;
        |           ^~~~~~~~~~

This patch causes GCC to interpret the code using the GNU dialect of the C++14
standard, as GCC 10 did by default, and reduces these errors to warnings.

diff --git a/makefile.linux_amd64_asm b/makefile.linux_amd64_asm
index 075c6a8..b7d73ee 100644
--- a/makefile.linux_amd64_asm
+++ b/makefile.linux_amd64_asm
@@ -11,6 +11,8 @@ ALLFLAGS=-m64 ${OPTFLAGS} -pipe \
 	-D_7ZIP_ASM \
 	$(LOCAL_FLAGS)
 
+ALLFLAGS_CPP=-std=gnu++14
+
 CXX=g++
 CC=gcc
 CC_SHARED=-fPIC
diff --git a/makefile.linux_any_cpu_gcc_4.X b/makefile.linux_any_cpu_gcc_4.X
index be093b5..c0f3120 100644
--- a/makefile.linux_any_cpu_gcc_4.X
+++ b/makefile.linux_any_cpu_gcc_4.X
@@ -10,7 +10,7 @@ ALLFLAGS=${OPTFLAGS} -pipe \
 	-D_7ZIP_LARGE_PAGES \
 	$(LOCAL_FLAGS)
 
-ALLFLAGS_CPP=-DENV_HAVE_GCCVISIBILITYPATCH -fvisibility=hidden -fvisibility-inlines-hidden
+ALLFLAGS_CPP=-DENV_HAVE_GCCVISIBILITYPATCH -std=gnu++14 -fvisibility=hidden -fvisibility-inlines-hidden
 
 CXX=g++
 CC=gcc
diff --git a/makefile.linux_x86_asm_gcc_4.X b/makefile.linux_x86_asm_gcc_4.X
index 1ac339b..6e736d9 100644
--- a/makefile.linux_x86_asm_gcc_4.X
+++ b/makefile.linux_x86_asm_gcc_4.X
@@ -12,7 +12,7 @@ ALLFLAGS=${OPTFLAGS} -pipe -m32 \
 	-D_7ZIP_ASM \
 	$(LOCAL_FLAGS)
 
-ALLFLAGS_CPP=-DENV_HAVE_GCCVISIBILITYPATCH -fvisibility=hidden -fvisibility-inlines-hidden
+ALLFLAGS_CPP=-DENV_HAVE_GCCVISIBILITYPATCH -std=gnu++14 -fvisibility=hidden -fvisibility-inlines-hidden
 
 CXX=g++
 CC=gcc
lue='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=55ab9a98c1f360501d92fcf882f6cbea9f4f83a1'>root</a>/<a href='/guix/log/manifest.scm?id=55ab9a98c1f360501d92fcf882f6cbea9f4f83a1'>manifest.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/manifest.scm?id=55ab9a98c1f360501d92fcf882f6cbea9f4f83a1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-06-24 08:51:43 -0400'>2024-06-24</span></td><td><a href='/guix/commit/manifest.scm?id=7f194852a46edfe47537d11bc39fb1dd20e18b37'>manifest: Streamline; add packages useful for patch review/submission.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2024-04-02 23:28:58 +0200'>2024-04-02</span></td><td><a href='/guix/commit/manifest.scm?id=049273bb8c1ea3d6402db646312acdc4abf4678a'>maint: Add perl to the manifest.</a><span class='msg-avail'>...</span></td><td>Janneke Nieuwenhuizen</td></tr>
<tr><td><span title='2024-01-31 09:07:25 +0100'>2024-01-31</span></td><td><a href='/guix/commit/manifest.scm?id=66dd088d3c3404f5cbe2d4d4c5e8f93d184703b0'>maint: Add gnupg to the manifest.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-09-18 22:12:39 +0200'>2023-09-18</span></td><td><a href='/guix/commit/manifest.scm?id=7bf12820f61179900f9d1f753acf91fd33724976'>maint: Support `guix shell' in Guix's git archive with manifest.scm.</a><span class='msg-avail'>...</span></td><td>Janneke Nieuwenhuizen</td></tr>