BUILD: rework makefile

- respect CXX, PKG_CONFIG, CXXFLAGS, LDFLAGS, LIBS, DESTDIR
- separate CXXFLAGS and LDFLAGS
- use pkg-config for detecting hexchat-plugin includedir
- install into system-wide plugin dir
- fix uninstall rule
Merge pull request #1 from hasufell/master

BUILD: rework makefile