package/heirloom-mailx: fix build failure due to gcc bug 101916

package/heirloom-mailx: fix build failure due to gcc bug 101916

The heirloom-mailx package exhibits gcc bug 101916 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_101916=y.
Also introduce HEIRLOOM_MAILX_CFLAGS as done for other packages and move
the already present -fPIC CFLAG to it.

Fixes:
http://autobuild.buildroot.net/results/911/911f5c024834741754102ff1bbb05c4a64c54a0b/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>