build: make ARM NEON check in configure.ac more strict
the check for NEON so far only checked if -mfpu=neon is understood by the compiler,
however, this is not enough:
(i) #include <arm_neon.h> should be checked
(ii) -mfpu=neon must be passed before CFLAGS because eventually the per-library CFLAGS
for NEON code in src/Makefile.am are passed to the compiler before the global CFLAGS
in case the build environment passes CFLAGS to configure and we try to set conflicting
CFLAGS option, the former take precedence; CFLAGS cannot be overridden
this does not fix
http://lists.freedesktop.org/archives/pulseaudio-discuss/2012-December/015570.html
but at least makes the build fail in configure (and not while compiling stuff)
and gives better diagnostics
Signed-off-by: Peter Meerwald <p.meerwald@bct-electronic.com>