re PR libfortran/16137 (Fortran compiler unable to produce executables as libfortran depends on C99 math functions)

PR libfortran/16137
* config.h.in (HAVE_POWF): Undefine.
* configure.ac: Check for 'powf' in library.
* configure: Regenerate.
* intrinsics/c99_functions.c (powf): New function.

From-SVN: r88128