Remove -fno-common compiler flag for OS X.

It doesn't allow the je_malloc_message and je_malloc_conf symbols to be
overridden when linking statically.
diff --git a/configure.ac b/configure.ac
index a09db7d..a72019e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -239,7 +239,7 @@
 default_munmap="1"
 case "${host}" in
   *-*-darwin*)
-	CFLAGS="$CFLAGS -fno-common"
+	CFLAGS="$CFLAGS"
 	abi="macho"
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_FREE], [ ])
 	RPATH=""