[svn-r7155] Reprioritize the compiler order used by autoconf. This allows us to
use more recent compilers when available instead of relying on older compilers by default: gfortran vs. g77 is one motivating example (autoconf prefers g77 for Fortran 77). Also remove MPD specific macros, allowing it to use confdb directly, so MPD can take advantage of this change as well. Reviewed by goodell.