core: Add ARM NEON optimized sample conversion code

final:
* includes some minor style fixes and build-time changes to allow
  building a single binary for neon and non-neon systems
v4:
* fix for sample length < 4
v3:
* convert from intrinsics to inline assembly
v2:
* load and store data with vld1/vld1q and vst1/vst1q, resp., to work
  around alignment issues of compiler-generated vldmia instruction
* remove redundant check for NEON flags

Ubuntu/Linaro gcc 4.6.3
arm-linux-gnueabi-gcc -O2 -mcpu=cortex-a8 -mfloat-abi=softfp -mfpu=neon

runtime on beagle-xm:

D: [pulseaudio] sconv_neon.c: checking NEON sconv_s16le_from_float
I: [pulseaudio] sconv_neon.c: NEON: 3754 usec.
I: [pulseaudio] sconv_neon.c: ref: 58594 usec.
D: [pulseaudio] sconv_neon.c: checking NEON sconv_s16le_to_float
I: [pulseaudio] sconv_neon.c: NEON: 1831 usec.
I: [pulseaudio] sconv_neon.c: ref: 10528 usec.
I: [pulseaudio] sconv_neon.c: Initialising ARM NEON optimized conversions.

conversion may be off by one for some samples due to rounding issues

configure.ac

@@ -317,6 +317,34 @@ case $host in
   ;;
 esac
 
+#### NEON optimisations ####
+AC_ARG_ENABLE([neon-opt],
+    AS_HELP_STRING([--enable-neon-opt], [Enable NEON optimisations on ARM CPUs that support it]))
+
+AS_IF([test "x$enable_neon_opt" != "xno"],
+    [save_CFLAGS="$CFLAGS"; CFLAGS="$CFLAGS -mfpu=neon"
+     AC_COMPILE_IFELSE(
+        AC_LANG_PROGRAM([], []),
+        [
+         HAVE_NEON=1
+         NEON_CFLAGS="-mfpu=neon"
+        ],
+        [
+         HAVE_NEON=0
+         NEON_CFLAGS=
+        ])
+     CFLAGS="$save_CFLAGS"
+    ],
+    [HAVE_NEON=0])
+
+AS_IF([test "x$enable_neon_opt" = "xyes" && test "x$HAVE_NEON" = "x0"],
+      [AC_MSG_ERROR([*** Compiler does not support -mfpu=neon])])
+
+AC_SUBST(HAVE_NEON)
+AC_SUBST(NEON_CFLAGS)
+AM_CONDITIONAL([HAVE_NEON], [test "x$HAVE_NEON" = x1])
+AS_IF([test "x$HAVE_NEON" = "x1"], AC_DEFINE([HAVE_NEON], 1, [Have NEON support?]))
+
 
 #### libtool stuff ####