heirloom-mailx: disable static build

heirloom-mailx: disable static build

heirloom-mailx implements its own version of getopt() that collides with the C
library when linking statically.

Fixes:
http://autobuild.buildroot.net/results/f67/f67eb63a9a8bb25fb0c071f1aac271e9798d95d1/
http://autobuild.buildroot.net/results/722/722f65c4fcc093d6a6a2200c06b2ecb3ee35bc06/
http://autobuild.buildroot.net/results/1cc/1cce267cc4b0a7d6bf5ff10f5429ae7dfe96960a/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>