Change sed calls to work on both MacOS X and Linux.

The syntax for the -i parameter is not compatible, so just write the
modified file temporarily and move it in place after the sed call.
Show a warning message in case some compile variables are set.

Currently warning is shown for:
- CFLAGS
- CXXFLAGS
- LDFLAGS
Add libxml2 to dependency build script and configure fontconfig to use it.