BUILD: makefile: do not pass warnings to VERBOSE_CFLAGS

BUILD: makefile: do not pass warnings to VERBOSE_CFLAGS

The VERBOSE_CFLAGS variable is used to report the CFLAGS used in the
output of "haproxy -vv". It currently contains all -Wxxx and -Wno-xxx
because there was no other possibility till now, but these warnings
are highly specific to the compiler version in use, are automatically
detected and do not bring value being presented here. Worse, by
encouraging users to copy-paste them, they can end up with warnings
that are not relevant to their build environment.

In addition, their presence there doesn't give indication of whether
or not they triggered, so they cannot vouch for the output code quality.

Better just not report them there. However, as part of VERBOSE_CFLAGS
they were also used to detect whether or not to rebuild, via build_opts,
so they still have to be explicitly mentioned there if we want to make
sure that changing warning options triggers a rebuild to see their
effect.

Now we can have more useful outputs like this one which indicate precisely
what to use to safely rebuild the executable:

  Build options :
    TARGET  = linux-glibc
    CC      = gcc
    CFLAGS  = -O2 -g -fwrapv
    OPTIONS = USE_OPENSSL=1 USE_LUA=1 USE_ZLIB= USE_SLZ=1 USE_DEVICEATLAS=1 USE_51DEGREES=1 USE_WURFL=1 USE_QUIC=1 USE_PROMEX=1 USE_PCRE=1
    DEBUG   = -DDEBUG_DONT_SHARE_POOLS -DDEBUG_MEMORY_POOLS -DDEBUG_EXPR -DDEBUG_STRICT=2 -DDEBUG_DEV -DDEBUG_MEM_STATS -DDEBUG_POOL_TRACING