configure: cleanup GLX_USE_TLS handling

Mesa requires ax_pthread_ok = yes, thus we can fold/rewrite the
conditional to follow the more common "if test" pattern.

No functional change intended.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/configure.ac b/configure.ac
index 56e91f0..799f5eb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1710,8 +1710,9 @@
     [GLX_USE_TLS=yes])
 AC_SUBST(GLX_TLS, ${GLX_USE_TLS})
 
-AS_IF([test "x$GLX_USE_TLS" = xyes -a "x$ax_pthread_ok" = xyes],
-      [DEFINES="${DEFINES} -DGLX_USE_TLS"])
+if test "x$GLX_USE_TLS" = xyes; then
+    DEFINES="$DEFINES -DGLX_USE_TLS"
+fi
 
 dnl Read-only text section on x86 hardened platforms
 AC_ARG_ENABLE([glx-read-only-text],