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
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=5488c0ac0659f65ea43cabfafa0c97cdd44fd811'>root</a>/<a href='/guix/log/gnu?id=5488c0ac0659f65ea43cabfafa0c97cdd44fd811'>gnu</a>/<a href='/guix/log/gnu/packages?id=5488c0ac0659f65ea43cabfafa0c97cdd44fd811'>packages</a>/<a href='/guix/log/gnu/packages/libffcall.scm?id=5488c0ac0659f65ea43cabfafa0c97cdd44fd811'>libffcall.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/libffcall.scm?id=5488c0ac0659f65ea43cabfafa0c97cdd44fd811&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-02-14 18:33:55 +0100'>2020-02-14</span></td><td><a href='/guix/commit/gnu/packages/libffcall.scm?id=caf43fd31ef5840abc76dd3e08d13628d681d1e4'>gnu: libffcall: Update to 2.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/libffcall.scm (libffcall): Update to 2.2.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-02-14 18:33:55 +0100'>2020-02-14</span></td><td><a href='/guix/commit/gnu/packages/libffcall.scm?id=ac906f14bcd683c7da2f94bbbbd7ee3469f1368f'>gnu: libffcall: Do not build the static libraries.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/libffcall.scm (libffcall)[arguments]: Add #:configure-flags.
</span></span></td><td>Marius Bakke</td></tr>