AC_COMPILE_IFELSE equivalent for CCAS

[D'Alessandro, Luke K]

Hi everyone.

We have some .S functionality that uses x86_64’s cmpxchg16b that we compile 
with CCAS. We’d like to test for architecture support for this instruction (for 
k1om and others). Ideally we would like to do this using an AC_COMPILE_IFELSE 
equivalent for raw .S code, rather than C code with inline asm (as done 
elsewhere in autoconf-archive) because we have some compiler toolchain 
issues—sometimes compilers don’t seem to realize that the instruction isn’t 
supported on the architecture when it is in inline asm.

Is there a straightforward way to deal with this as a COMPILE test? RUN tests 
are problematic because the code will be running on a coprocessor (quasi-cross 
compiled).

Thanks,
Luke