m4.include/mc-cflags.m4: report real compiler in MC_CHECK_ONE_CFLAG instead of 'gcc'

Example use: CC=clang ./configure

Before the patch configure reported 'gcc' is being used:

    checking whether gcc accepts -Wcomment... yes
    checking whether gcc accepts -Wdeclaration-after-statement... yes
    checking whether gcc accepts -Wfloat-equal... yes
    checking whether gcc accepts -Wformat... yes

(config.log used proper ${CC})

After the patch it lies a bit less:

    checking whether clang accepts -Wcomment... yes
    checking whether clang accepts -Wdeclaration-after-statement... yes
    checking whether clang accepts -Wfloat-equal... yes
    checking whether clang accepts -Wformat... yes

Reported-by: Agostino Sarubbo <ago@gentoo.org>
Gentoo-bug: http://bugs.gentoo.org/449752
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>