Only run pkg-config once in Makefile, added EXTRA linker vars

Only run pkg-config once in Makefile, added EXTRA linker vars

* Only run the pkg-config --cflags command once (instead of every .c file)
* Added EXTRA_LDFLAGS and EXTRA_LDLIBS vars
* Removed tabs in the middle of commands to make it easier to manually re-run
  commands manually (e.g. if a compilation fails)
* Replaced `cd dir; git command` in favor of $(shell git -C dir command)
* Minor version format change: dirty trees just append "+" instead of "-dirty"
  and if git describe fails, it now sets the version to "v0.1-nogit"

Also, don't ignore .gitignore files.