gc: fix musl build. (patch from Alpine.)

srcpkgs/gc/patches/fix-boehm-gc.patch

@@ -0,0 +1,11 @@
+--- os_dep.c
++++ os_dep.c
+@@ -26,7 +26,7 @@
+ #   define __KERNEL__
+ #   include <asm/signal.h>
+ #   undef __KERNEL__
+-# else
++# elif defined(__GLIBC__)
+     /* Kernels prior to 2.1.1 defined struct sigcontext_struct instead of */
+     /* struct sigcontext.  libc6 (glibc2) uses "struct sigcontext" in     */
+     /* prototypes, so we have to include the top-level sigcontext.h to    */

srcpkgs/gc/template

@@ -5,6 +5,10 @@ revision=1
 build_style=gnu-configure
 hostmakedepends="automake pkg-config libtool"
 makedepends="libatomic_ops-devel"
+case "$XBPS_TARGET_MACHINE" in
+*-musl)
+	CFLAGS='-D_GNU_SOURCE -DNO_GETCONTEXT -DUSE_MMAP'
+esac
 short_desc="A garbage collector for C and C++"
 maintainer="Juan RP <xtraeme@voidlinux.eu>"
 homepage="http://www.hboehm.info/gc/"