This is an idea I got when debugging libm test cases, and finding that
the compiler had neatly inlined pow(), sin() and various other calls.
Obviously for general code this behavior is just fine, but not when
either building libm itself, or its test cases!
Of course the easy way out is to use -fno-builtin, turning off the
compiler's builtins completely. But on the other hand, it would be
perfectly fine to have it e.g. inline or optimize non-math related calls
like memcpy(), both while building libm itself, or its tests.
Therefore I made a list of math functions that are announed in math.h
and complex.h, and added a separate math-builtins.mk which disables each
of those indidivually, using -fno-builtin-xxx.