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
f62'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='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=62cf682938b8f7cb0512495961935dca9fe5ef62'>root</a>/<a href='/guix/log/doc?id=62cf682938b8f7cb0512495961935dca9fe5ef62'>doc</a>/<a href='/guix/log/doc/images?id=62cf682938b8f7cb0512495961935dca9fe5ef62'>images</a>/<a href='/guix/log/doc/images/service-graph.dot?id=62cf682938b8f7cb0512495961935dca9fe5ef62'>service-graph.dot</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/doc/images/service-graph.dot?id=62cf682938b8f7cb0512495961935dca9fe5ef62&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-04-14 10:29:07 +0200'>2024-04-14</span></td><td><a href='/guix/commit/doc/images/service-graph.dot?id=076670f7acb740e2bfbd8e3c8ef1be5221ed75c5'>doc: Use "dejavu sans" instead of "Helvetica" or "sans" in dot images.</a><span class='msg-avail'>...<span class='msg-tooltip'>Font-dejavu is (now) an input of fontconfig.  Specifying "dejavu sans" makes
generated images reproducible even when other fonts are installed (notably
font-google-noto).

* doc/images/bootstrap-graph.dot,
doc/images/bootstrap-packages.dot,
doc/images/coreutils-bag-graph.dot,
doc/images/coreutils-graph.dot,
doc/images/gcc-core-mesboot0-graph.dot,
doc/images/service-graph.dot,
doc/images/shepherd-graph.dot: Use fontname = "dejavu sans".
* doc/guix.texi (Full-Source Bootstrap): Update gcc-core-mesboot0.dot recipe
accordingly.

Change-Id: If21d7d39d45c66de5bceafb7b825a057d540ee50
</span></span></td><td>Janneke Nieuwenhuizen</td></tr>