pixman: explicitly disable openmp in native builds

pixman: explicitly disable openmp in native builds

* on some hosts (e.g. on my gentoo after recent update) it detects OpenMP in do_configure:
  Run-time dependency OpenMP found: YES 4.5
  but then fails in do_compile as shown in:
  http://errors.yoctoproject.org/Errors/Details/754632/

  gcc  -o test/fetch-test test/fetch-test.p/fetch-test.c.o -LTOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/usr/lib -LTOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/lib -Wl,--as-needed -Wl,--no-undefined -Wl,--enable-new-dtags -Wl,-rpath-link,TOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/usr/lib -Wl,-rpath-link,TOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/lib -Wl,-rpath,TOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/usr/lib -Wl,-rpath,TOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/recipe-sysroot-native/lib -Wl,-O1 '-Wl,-rpath,$ORIGIN/../pixman' -Wl,-rpath-link,TOPDIR/tmp-glibc/work/x86_64-linux/pixman-native/0.42.2/build/pixman -Wl,--start-group test/libtestutils.a pixman/libpixman-1.so.0.42.2 -pthread -fopenmp -fopenmp -lm -Wl,--end-group
  gcc: fatal error: cannot read spec file ‘libgomp.spec’: No such file or directory
  compilation terminated.

  it's only used in tests, so should be safe to disable for native builds

* the check in meson uses /usr/include/omp.h which is provided by libomp
  even when openmp support is disabled in native gcc in gentoo this happned
  after switching from 17.1 profile to 23.0 which doesn't include openmp
  USE flag by default, if you later run depclean it will uninstall libomp
  as well which will fix this conflict in meson's OpenMP autodetection

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>