This directory holds third party libraries as git subtrees

If the directories are empty, the makefile based build will try
to use system libraries instead.

The third party modules should be already here, but here is a
reference to where they came from:

git subtree add --prefix=thirdparty/jbig2dec https://github.com/ArtifexSoftware/jbig2dec.git jbig2dec-0.17 --squash
git subtree add --prefix=thirdparty/mujs https://github.com/ArtifexSoftware/mujs.git 1.0.6 --squash
git subtree add --prefix=thirdparty/freetype https://github.com/ArtifexSoftware/thirdparty-freetype2.git VER-2-10-0-mupdf-2 --squash
git subtree add --prefix=thirdparty/harfbuzz https://github.com/ArtifexSoftware/thirdparty-harfbuzz.git 1.9.0 --squash
git subtree add --prefix=thirdparty/libjpeg https://github.com/ArtifexSoftware/thirdparty-libjpeg.git artifex --squash
git subtree add --prefix=thirdparty/lcms2 https://github.com/ArtifexSoftware/thirdparty-lcms2.git lcms2mt --squash
git subtree add --prefix=thirdparty/openjpeg https://github.com/ArtifexSoftware/thirdparty-openjpeg.git master --squash
git subtree add --prefix=thirdparty/zlib https://github.com/ArtifexSoftware/thirdparty-zlib.git 1.2.11 --squash
git subtree add --prefix=thirdparty/curl https://github.com/ArtifexSoftware/thirdparty-curl.git artifex --squash
git subtree add --prefix=thirdparty/freeglut https://github.com/ArtifexSoftware/thirdparty-freeglut.git master --squash