package/gawk: add optional support for libsigsegv

package/gawk: add optional support for libsigsegv

When libsigsegv was compiled before, gawk will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/gawk | grep NEEDED
  0x0000000000000001 (NEEDED)             Shared library: [libsigsegv.so.2]
[...]

The build system offers no option to en-/disable libsigsegv support:
http://git.savannah.gnu.org/cgit/gawk.git/tree/m4/libsigsegv.m4

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>