autoconf: Check for posix_memalign

Rather than just defining HAVE_POSIX_MEMALIGN on Linux, check whether
the function exists on all platforms and define the macro if it is.

configure.ac

@@ -85,7 +85,7 @@ linux*)
 if test "x$GCC" = xyes; then
     DEFINES="$DEFINES -D_POSIX_SOURCE -D_POSIX_C_SOURCE=199309L -D_BSD_SOURCE"
 fi
-    DEFINES="$DEFINES -D_SVID_SOURCE -D_GNU_SOURCE -DPTHREADS -DHAVE_POSIX_MEMALIGN"
+    DEFINES="$DEFINES -D_SVID_SOURCE -D_GNU_SOURCE -DPTHREADS"
     ;;
 solaris*)
     DEFINES="$DEFINES -DPTHREADS -DSVR4"
@@ -325,6 +325,9 @@ dnl has it in libc), or if libdl is needed to get it.
 AC_CHECK_FUNC([dlopen], [],
     [AC_CHECK_LIB([dl], [dlopen], [DLOPEN_LIBS="-ldl"])])
 
+dnl See if posix_memalign is available
+AC_CHECK_FUNC([posix_memalign], [DEFINES="$DEFINES -DHAVE_POSIX_MEMALIGN"])
+
 dnl SELinux awareness.
 AC_ARG_ENABLE([selinux],
     [AS_HELP_STRING([--enable-selinux],