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
 msg</option>
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<div class='path'>path: <a href='/guix/log/?id=f40234e225dcb6bfc51c05cf51fd01841e0f5dbd'>root</a>/<a href='/guix/log/build-aux?id=f40234e225dcb6bfc51c05cf51fd01841e0f5dbd'>build-aux</a>/<a href='/guix/log/build-aux/test-env.in?id=f40234e225dcb6bfc51c05cf51fd01841e0f5dbd'>test-env.in</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/build-aux/test-env.in?id=f40234e225dcb6bfc51c05cf51fd01841e0f5dbd&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-12-22 10:53:19 +0100'>2021-12-22</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=9ebc9ca070a21ba0aeb3f61202a8071bb250ccc8'>tests: Move keys into ./tests/keys/ and add a third ed25519 key.</a><span class='msg-avail'>...</span></td><td>Attila Lendvai</td></tr>
<tr><td><span title='2021-04-20 12:13:16 +0200'>2021-04-20</span></td><td><a href='/guix/commit/build-aux/test-env.in?id=d95168321f4a9bf6857b598da0a183b45a868d54'>import: Remove Nix importer.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>