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
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=7235ce224e3d20b5f73474cac2d3b34b50d754a5'>root</a>/<a href='/guix/log/gnu?id=7235ce224e3d20b5f73474cac2d3b34b50d754a5'>gnu</a>/<a href='/guix/log/gnu/system?id=7235ce224e3d20b5f73474cac2d3b34b50d754a5'>system</a>/<a href='/guix/log/gnu/system/file-systems.scm?id=7235ce224e3d20b5f73474cac2d3b34b50d754a5'>file-systems.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/system/file-systems.scm?id=7235ce224e3d20b5f73474cac2d3b34b50d754a5&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-20 08:30:35 -0400'>2020-05-20</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=b460ba7992a0b4af2ddb5927dcf062784539ef7b'>bootloader: grub: Allow booting from a Btrfs subvolume.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-05-20 08:30:34 -0400'>2020-05-20</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=fa35fb58c84d1c1741e4e63c0b37074e35ed2a61'>file-systems: Add helpers for parsing the options string into an alist.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-04-27 14:14:47 +0200'>2020-04-27</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=2ef4d273d13faccbc9e11a74cfdbd0a9f4f1c898'>file-systems: mount the PID cgroup filesystem.</a><span class='msg-avail'>...</span></td><td>Jakub Kądziołka</td></tr>
<tr><td><span title='2020-03-02 23:24:47 -0500'>2020-03-02</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=99e676db43389af3a9e4c21734987c086fcaa8ed'>file-systems: Add a 'file-system-device-&gt;string' procedure.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2019-11-04 23:35:32 +0100'>2019-11-04</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=625bdf09d344302ec2d5da7f35fe35ca1d128a93'>file-systems: Mount /var/run/nscd read-write in containers.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-12 23:07:42 +0200'>2019-09-12</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=5ccec77176b7e0c67ed58c8849e5e76f3dd79a88'>file-systems: Add /var/run/nscd to '%network-file-mappings'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-19 11:53:47 +0200'>2019-07-19</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=b41c7beb0b5b7a16656d6acf53f77eaf2a58e125'>file-systems: Use 'no-atime' for %IMMUTABLE-STORE.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-14 11:16:59 +0200'>2018-06-14</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=c45477d2a1a651485feede20fe0f3d15aec48b39'>install: Use (guix store database) instead of 'guix-register'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-05-28 13:24:11 +0200'>2018-05-28</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=a5acc17a3c10a3779b5b8b1a2565ef130be77e51'>file-systems: Remove 'title' field and add &lt;file-system-label&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-05-19 00:14:52 +0200'>2018-05-19</span></td><td><a href='/guix/commit/gnu/system/file-systems.scm?id=a48d34504181e2f14cef3a5514d2319e60453a79'>file-systems: Do not export &lt;file-system&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>