Added ENABLE_XML_PARSER compile time option.

The default is true.

If ENABLE_XML_PARSER is true then SDL2_bmfont is compiled with the
ability to parse XML files using libxml2.

If ENABLE_XML_PARSER is false then SDL2_bmfont does NOT compile
with the ability to parse XML files. However, we no longer depend
on libxml2. Since we provide an internal text parser this can be
good for portability.

This required reworking CMakeLists.txt, adding the ENABLE_XML_PARSER
compile flag, and using it in a preprocessor case when we try to
load the .fnt file (in src/bmfont.c).