build-sys: fix ENABLE_ALL

Signed-off-by: Karel Zak <kzak@redhat.com>
build-sys: add --enable-pylibmount

 * globally disable all Python bindings by --without-python
 * use --with-python=<version> to specify Python version
 * --enable-pylibmount to override --disable-all-programs

Reported-by: Ruediger Meier <sweet_f_a@gmx.de>
Signed-off-by: Karel Zak <kzak@redhat.com>