[CI] Find a way to avoid certain CLI flags not supported by compilers

[jimklimov]

A few options probed in m4/nut_compiler_family.m4 are not really supported by some versions of compilers that the NUT CI build farm uses (whether by front-ends like gcc or back-ends like cc1) and are reported as problematic during "real builds". During the test operations done in the m4 script however, they seem fine and are accepted - only to begin complaining when e.g. C++11 support is probed just after.

They seem to also be reported as errors when -Werror mode is used (so warnings are fatal) but were not see to cause actual build failures, just confusion about why they failed (and noise in the log or terminal).

Seen with -Wno-reserved-identifier and -Wno-unknown-warning-option on gcc-4.x.

[jimklimov]

It seems the problem was between AX_CHECK_COMPILE_FLAG() and AC_LANG_PUSH()/AC_LANG_POP() and/or AC_MSG_CHECKING(), in that they saved -Werror from fourth argument of the former macro into the resulting CFLAGS or CXXFLAGS and seem to have ignored the "if failed" third argument of the latter.

Maybe not all of this analysis is spot-on, but anyhow the proposed change to raise a flag and apply the update or revert of the standard variable after the lang pop did help ignore the unrecognized compiler parameters.

Another part of the issue is that the initial check seems to ignore -Werror and succeeds compilation with whatever -W* arguments, and so the inner check (if first step was successful, try to link and/or run) is still needed. The initial check does fail for wholly unknown arguments like -Q* not supported by some compiler versions.

[jimklimov]

It still misfires for some cases, e.g.:

configure:11094: Starting check compile flag for '-Wno-reserved-identifier'; now CFLAGS='-g -O2' and CXXFLAGS='-g -O2'
configure:11107: checking whether C compiler accepts -Wno-reserved-identifier
configure:11127: /usr/lib/ccache/gcc-4.4.4 -std=gnu99 -c -g -O2  -Wno-reserved-identifier  conftest.c >&5
configure:11127: $? = 0
configure:11145: result: yes
configure:11150: checking whether the flag '-Wno-reserved-identifier' is still supported in CC linker mode
configure:11204: /usr/lib/ccache/gcc-4.4.4 -std=gnu99 -o conftest -Werror -g -O2 -Wno-reserved-identifier -Werror -Werror=implicit-function-declaration    conftest.c  >&5
configure:11204: $? = 0
configure:11204: ./conftest
configure:11204: $? = 0
configure:11216: result: yes
configure:11238: yes for C '-Wno-reserved-identifier'; now CFLAGS=-g -O2 -Wno-reserved-identifier

...

configure:11401: Starting check compile flag for '-fdiagnostics-color=always'; now CFLAGS='-g -O2 -Wno-reserved-identifier' and CXXFLAGS='-g -O2'
configure:11414: checking whether C compiler accepts -fdiagnostics-color=always
configure:11434: /usr/lib/ccache/gcc-4.4.4 -std=gnu99 -c -g -O2 -Wno-reserved-identifier  -fdiagnostics-color=always  conftest.c >&5
configure:11434: $? = 0
configure:11452: result: yes
configure:11457: checking whether the flag '-fdiagnostics-color=always' is still supported in CC linker mode
configure:11511: /usr/lib/ccache/gcc-4.4.4 -std=gnu99 -o conftest -Werror -g -O2 -Wno-reserved-identifier -fdiagnostics-color=always -Werror -Werror=implicit-function-declaration    conftest.c >&5
cc1: error: unrecognized command line option "-fdiagnostics-color=always"
cc1: warnings being treated as errors
cc1: error: unrecognized command line option "-Wno-reserved-identifier"
configure:11511: $? = 1
configure: program exited with status 1
configure: failed program was:
...