Use CFLAGS for testcases on AIX

On AIX, one can compile applications either in 32bit or 64bit.
With GCC, this is done by using: -maix64 or -maix32 (default).
Thus, when building & testing Swig in 64bit, the -maix64 option must be
passed to all calls to gcc.

Fixes #1923

configure.ac

@@ -289,6 +289,7 @@ case $host in
     else PLATCFLAGS=
       #    else PLATCFLAGS="-errtags=yes" # Need more work as C examples use ld for linking
     fi;;
+  *-*-aix*) PLATCFLAGS="$CFLAGS";;
   *) PLATCFLAGS=
 esac
 AC_MSG_RESULT($PLATCFLAGS)