Keep "-lmingwex" and "-lmoldname" in linker flags for MinGW

Keep "-lmingwex" and "-lmoldname" in linker flags for MinGW

It is needed when building Octave targeting Windows. Without it, the
implementation of "casin" from the Windows runtime is used instead of
the replacement from MinGW-w64.  That means that results for the
"casin" function are wrong without that change.
* lib/autoconf/fortran.m4: Do not remove "-lmingwex" or "-lmoldname"
from the linker flags when targeting MinGW.  Removing "-lmingwex" causes
an incorrect linker resolution for functions like "casin" for projects
that consist of Fortran and C++ sources.  Removing "-lmoldname" causes
issues for Fortran projects that try to use POSIX-like C functions
without a '_' symbol prefix when targeting MinGW.