build: Add support for --enable-relocatable in mingw builds on Cygwin hosts.

build: Add support for --enable-relocatable in mingw builds on Cygwin hosts.

Reported by Michele Locati <michele@locati.it> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-09/msg00033.html>.

* gettext-runtime/configure.ac: Determine bindir_c_make.
* gettext-runtime/src/Makefile.am (*_CFLAGS): Fix value of INSTALLDIR.
* gettext-tools/configure.ac: Determine bindir_c_make, pkglibdir_c_make.
* gettext-tools/src/Makefile.am (*_CPPFLAGS): Fix value of INSTALLDIR.