media-gfx/gimp: Fix suppressing system CFLAGS on configuration tests
During configuration tests CPU support of SSE2 and SSE4.1 in configure.ac file
the system env variable CFLAGS parameters are suppressed by addition of -msse2
and -msse4.1 that results in the GIMP will be compiled with their support
but the compilation will fail if system CFLAGS has SSE2 and SSE4.1 disabled
(i.e. -mno-sse2 and -mno-sse4.1 is used).
Closes: https://bugs.gentoo.org/702554
Signed-off-by: Sergey Torokhov <torokhov-s-a@yandex.ru>
Closes: https://github.com/gentoo/gentoo/pull/13992
Signed-off-by: Joonas Niilola <juippis@gentoo.org>