BUILD: add an AIX 5.2 (and later) target.

It's always a real pain to build on AIX and I constantly lose my flags,
so let's store them once for all in the Makefile.
diff --git a/README b/README
index ca6312d..06187a6 100644
--- a/README
+++ b/README
@@ -29,6 +29,7 @@
   - solaris     for Solaris 8 or 10 (others untested)
   - freebsd     for FreeBSD 5 to 8.0 (others untested)
   - openbsd     for OpenBSD 3.1 to 4.6 (others untested)
+  - aix52       for AIX 5.2
   - cygwin      for Cygwin
   - generic     for any other OS.
   - custom      to manually adjust every setting
@@ -107,6 +108,8 @@
 AIX 5.3 is known to work with the generic target. However, for the binary to
 also run on 5.2 or earlier, you need to build with DEFINE="-D_MSGQSUPPORT",
 otherwise __fd_select() will be used while not being present in the libc.
+If you get build errors because of strange symbols or section mismatches,
+simply remove -g from DEBUG_CFLAGS.
 
 
 2) How to install it