Use environment for mips feature detection

[matoro]

The -march= option is perfectly happy to emit code to run on a processor different than the one on which it is being compiled. This results in misdetection of mips features because the test compiles specify that a given extension should be emitted, but this does not check whether or not this corresponds to the subarchitecture targeted in CFLAGS by the rest of the build.

$ echo "void main(void){ __asm__ volatile(\"punpcklhw \$f0, \$f0, \$f0\"); }" > test.c
$ CFLAGS="-march=loongson3a" make test
cc -march=loongson3a    test.c   -o test
$ ./test
Illegal instruction
$ CFLAGS="-march=native" make -B test
cc -march=native    test.c   -o test
/tmp/ccLbeyM1.s: Assembler messages:
/tmp/ccLbeyM1.s:25: Error: opcode not supported on this processor: octeon2 (mips64r2) `punpcklhw $f0,$f0,$f0'
make: *** [<builtin>: test] Error 1

This leads to -march=loongson3a getting appended to CFLAGS, which may conflict with previously specified -march= levels for the build, or other options. Calling make in the test will use whatever CC/CFLAGS are specified in the environment to determine whether the actual compile command line to be used in the build supports these features.

[LodewijkIVX]

;-)