iOS: Disable NEON until LLVM handles the GAS syntax of the draw-helpers

The LLVM version in Xcode 4.5 and below does not handle the GNU
assembler syntax in the pixman assembly file.

A possible alternative workaround is to include a preprocessed assembly
file using https://github.com/hollylee/gas-preprocessor.

Change-Id: Id95add669c60d3a7da823e5975afdd1f88f71977
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

configure

@@ -4032,7 +4032,8 @@ fi
 
 # detect neon support
 if [ "$CFG_ARCH" = "arm" ] && [ "${CFG_NEON}" = "auto" ]; then
-    if compileTest unix/neon "neon"; then
+    # The iOS toolchain has trouble building the pixman NEON draw-helpers
+    if [ "$XPLATFORM_IOS" != "yes" ] && compileTest unix/neon "neon"; then
 	CFG_NEON=yes
     else
 	CFG_NEON=no