Revert to the pre-8.2 method of probing for libm, that is, always

Revert to the pre-8.2 method of probing for libm, that is, always
include it if it links properly.  It seems too risky to assume that
standard functions like pow() are not special-cased by the compiler.
Per report from Andreas Lange that build fails on Solaris cc compiler
with -fast.  Even though we don't consider that a supported option,
I'm worried that similar issues will arise with other compilers.