package/pixman: fix build failure due to gcc bug 101737

package/pixman: fix build failure due to gcc bug 101737

The pixman package exhibits gcc bug 101737 when built for the SH4
architecture with optimization enabled, which causes a build failure.

As done for other packages in Buildroot work around this gcc bug by
setting optimization to -O0 if BR2_TOOLCHAIN_HAS_GCC_BUG_101737=y.
Also let's add PIXMAN_CFLAGS and pass the Codesourcery work around CFLAGS
to it for consistency like we do for the rest of the packages.

Fixes:
http://autobuild.buildroot.net/results/b20/b20869bbb48edb1f0a847ea9e2e1a0462d6350be/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>