As a result of http://www.openwall.com/lists/musl/2016/10/22/3
it has been shown that musl's current compiler sanity checks
are inadequate. This patch checks if the compiler as a whole
works properly, and moves the CPPFLAGS/CFLAGS/LDFLAGS sanity
checks after the 'compiler works' check.

Patch and patch signature attached

Thanks,
  Laine Gholson