Revert "configure - don't add -mfpu-neon on arm - this breaks building for armv6/4..."

This reverts commit c94252a14c.

configure.ac

@@ -510,6 +510,8 @@ case $host_cpu in
   arm*)
     build_cpu_neon="yes"
     AC_MSG_CHECKING([whether to use NEON instructions])
+    CFLAGS_save="${CFLAGS}"
+    CFLAGS="${CFLAGS} -mfpu=neon"
     AC_TRY_COMPILE([#include <arm_neon.h>],
        [asm volatile ("vqadd.u8 d0, d1, d0\n")],
        [
@@ -521,6 +523,7 @@ case $host_cpu in
         AC_MSG_RESULT([no])
         build_cpu_neon="no"
        ])
+    CFLAGS="${CFLAGS_save}"
     ;;
 esac