Fix OpenMP not supported case

Fix OpenMP not supported case

PIXMAN_LINK_WITH_ENV did not fail unless -Wall -Werror is used.
So even when the compiler did not support OpenMP, USE_OPENMP was defined.
Fix that by running the second OpenMP test only when first AC_OPENMP find supported

configure tested in the cases :
gcc without libgomp support, no openmp option, --enable-openmp and --disable-openmp
gcc with libgomp support, no openmp option, --enable-openmp and --disable-openmp

Not tested with autoconf version not knowing openmp (<2.62)

Warn when --enable-openmp is requested but no support is found

Signed-off-by: Gilles Espinasse <g.esp@free.fr>