Build Expat and GLM statically as dependencies from 3rdparty submodules

[devshgraphicsprogramming]

#253 will need GLM and we'll need an XML parser to implement a mitsuba loader for #303 .

We can achieve some tranquility as the mission to statically link CEGUI and its dependencies will be 40% accomplished (only Freetype2, GLEW and PCRE will hold us up).

Expat

CEGUI uses: https://github.com/libexpat/libexpat/releases/tag/R_2_0_1
But see if we can bump to https://github.com/libexpat/libexpat/releases/tag/R_2_1_1 or even https://github.com/libexpat/libexpat/releases/tag/R_2_2_6 without upsetting CEGUI

We should grab just the ./expat subdirectory from the repo (if possible) as the rest is travisCI crap.
It appears to have a working CMakeLists.txt

Obviously don't build expat if irr::ext::cegui nor irr::ext::mistubaloader is not being built (default bahviour with dependencies I guess?).

GLM

Use https://github.com/g-truc/glm/releases/tag/0.9.9.5
Unless CEGUI or GLI moan.

I guess glm will build by default to support the built-in loading of DDS and KTX.

@manhnt9 , @Crisspl , @Florastamine any takers?

[devshgraphicsprogramming]

expat 2.2.6 looks fine

[devshgraphicsprogramming]

committing a work-around for CEGUI's build system of death