BUILD: makefile: add _LINUX_SOURCE_COMPAT to build on AIX-51

Not tested on later versions, but at least there _LINUX_SOURCE_COMPAT
must be defined to access the CMSG_SPACE() and CMSG_LEN() macros.
diff --git a/Makefile b/Makefile
index b3a8e7a..38a60cd 100644
--- a/Makefile
+++ b/Makefile
@@ -382,7 +382,7 @@
 ifeq ($(TARGET),aix51)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_LIBCRYPT USE_OBSOLETE_LINKER)
-  TARGET_CFLAGS   = -Dss_family=__ss_family -Dip6_hdr=ip6hdr -DSTEVENS_API
+  TARGET_CFLAGS   = -Dss_family=__ss_family -Dip6_hdr=ip6hdr -DSTEVENS_API -D_LINUX_SOURCE_COMPAT
   DEBUG_CFLAGS    =
 endif