Add functionality to replace libm and libquadmath functions (#405)

With this patch, libm functions and libquadmath functions can be replaced with sleef functions by defining SLEEF_REPLACE_LIBM_FUNCS and SLEEF_REPLACE_LIBQUADMATH_FUNCS macros before including sleef.h and sleefquad.h, respectively. The replaced libm functions can be automatically vectorized with gcc on x86_64.