Fix all cmake issues related to building for win32 with mxe.
Add support for ./installdeps win32 on Debian/Ubuntu using the mxe apt
repository.
Details:
* default to ENABLE_LTO=OFF on mxe
* set SFML_STATIC_LIBRARIES=TRUE on mxe
* fix the libintl/gettext checking logic
* fix the git dependencies submodule checking logic, and check that it
triggers for mxe
* revert FindSFML.cmake to upstream version, works fine with mxe
* make 'win32' an alias for the 'MinGW-w64-i686' target for
./installdeps, add support for mxe apt repository for Debian/Ubuntu
* change bin2c related cmake code to compile the tool on the host always
even when cross-compiling
* copy src/win32/res/VBA.ico to src/wx/icons since src/win32 has been
removed
TODO: generic mxe installer for other unix-likes