Check if NEON extension are actually available

[jpalus]

User may pass own compiler flags to configure which override those provided by project through automake. Therefore it is possible for user on ARM platform to pass CXXFLAGS=-mfpu=vfp which will effectively disable NEON even though used compiler supports -mfpu=neon (since user supplied flags take precedence compiler invocation will use flags: -mfpu=neon -mfpu=vfp). Instead of checking whether compiler supports -mfpu=neon flag, check if NEON extensions are available by checking if __ARM_NEON is defined when compiling with -mfpu=neon and user supplied flags combined.