libtool: Fix MSVC cl.exe .exp extension collision

libtool: Fix MSVC cl.exe .exp extension collision

MSVC's compiler uses the .exp entension for their own export format,
which causes a build failure when a subsequent build is run. The
MSVC compiler, cl.exe, errors for an invalid file format. This is fixed
by using a similar extension name to avoid the naming collision.

Reported: https://savannah.gnu.org/support/?111157

* build-aux/ltmain.in: Replace .exp extension with .expsym.
* m4/libtool.m4: Replace .exp extension with .expsym.