perl: fix musl compatibility

- Add patch to inhibit the libm IEEE math switch on musl
 - Add postprocessing for config.sh to the Makefile to fixup
   defines for musl, idea taken
   from http://patchwork.openembedded.org/patch/91707/

This is the least invasive approach I could come up to fix the build
failure asap. Another possibility is maintaining yet another set of
musl specific architecture config files, but I think that this introduces
a lot of maintenance overhead.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

lang/perl/patches/100-musl-compat.patch

@@ -0,0 +1,11 @@
+--- a/pp.c
++++ b/pp.c
+@@ -43,7 +43,7 @@ extern Pid_t getpid (void);
+  * Some BSDs and Cygwin default to POSIX math instead of IEEE.
+  * This switches them over to IEEE.
+  */
+-#if defined(LIBM_LIB_VERSION)
++#if defined(LIBM_LIB_VERSION) && (defined(__GLIBC__) || defined(__UCLIBC__))
+     _LIB_VERSION_TYPE _LIB_VERSION = _IEEE_;
+ #endif
+