Fix tests/Makefile (#62)
Fix tests/Makefile (#62)

- Use CXX/CXXFLAGS as this is c++ code and CFLAGS/CPPFLAGS are unrelated
- Allow flags to be provided by the environment
- Use pkg-config for gtest discovery instead of using nonportable
  hardcoded flags
- Name target after the file it produces
brofield/simpleini,brofield/simpleini