Thomas Petazzoni authored Aug 06, 2016 and Arun Raghavan committed Aug 10, 2016

The current architecture detection, based on the "host_cpu" part of the
tuple does not work properly for a number of reason:

 - The code assumes that if host_cpu starts with "arm" then ARM
   instructions are available, which is incorrect. Indeed, Cortex-M
   platforms can run Linux, they are ARM platforms (so host_cpu = arm),
   but they don't support ARM instructions: they support only the
   Thumb-2 instruction set.

 - The armv7 case is also not very useful, as it is not standard at all
   to pass armv7 as host_cpu even if the host system is actually ARMv7
   based.

 - For the same reason, the armv8 case is not very useful: ARMv8 is
   AArch64, and there is already a separate case to handle this
   architecture.

So, this commit moves away from a host_cpu based logic, and instead
tests using AC_CHECK_DECLS() the built-in definitions of the compiler:

 - If we have __ARM_ARCH_ISA_ARM defined, then it's an ARM processor
   that supports the ARM instruction set (this allows to exclude Thumb-2
   only processors).

 - If we have __ARM_ARCH_7A__, then we have an ARMv7-A processor, and
   we can enable the corresponding optimizations

 - Same for __aarch64__, __i386__ and __x86_64__.

In addition, we remove the AC_MSG_ERROR() that makes the build fail for
all architectures but the ones that are explicitly supported. Indeed,
webrtc-audio-processing builds just fine for other architectures (tested
on MIPS), it's just that none of the architecture-specific optimizations
will be used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>