build: Add support for --enable-relocatable in mingw builds on Cygwin hosts, part 2.

build: Add support for --enable-relocatable in mingw builds on Cygwin hosts, part 2.

Reported by Michele Locati <michele@locati.it> in
<https://savannah.gnu.org/bugs/?66319>.

* gettext-tools/configure.ac: Invoke gl_CONFIGMAKE_PREP and fix up the values of
pkgdatadir, pkgincludedir, pkglibdir, pkglibexecdir.
Invoke gl_BUILD_TO_HOST_LIBDIR, gl_BUILD_TO_HOST_LIBEXECDIR,
gl_BUILD_TO_HOST_PKGDATADIR, gl_BUILD_TO_HOST_PKGLIBDIR.
* gettext-tools/src/Makefile.am (pkgdatadir, pkglibdir, pkglibexecdir): Remove
variables.
(jardir): Simplify.
(jardir_c_make, projectsdir_c_make): New variables.
(*_CPPFLAGS): Fix value of BISON_LOCALEDIR, GETTEXTJAR, LIBDIR, LIBEXECDIR,
GETTEXTDATADIR, PROJECTSDIR.