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>

package/heirloom-mailx/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_HEIRLOOM_MAILX
 	bool "heirloom-mailx"
 	# uses fork()
 	depends on BR2_USE_MMU
+	depends on !BR2_PREFER_STATIC_LIB
 	help
 	  Heirloom mailx (previously known as nail) is a mail user
 	  agent for Unix systems.
@@ -13,3 +14,7 @@ config BR2_PACKAGE_HEIRLOOM_MAILX
 	  cram-md5), SMTPS, MIME encoding of attachements, and more.
 
 	  http://heirloom.sourceforge.net/mailx.html
+
+comment "heirloom-mailx needs a toolchain w/ dynamic library"
+	depends on BR2_PREFER_STATIC_LIB
+	depends on BR2_USE_MMU