Use $(CXX) in ANN build instead of hardcoded g++

CXX is an implicit make variable that always has a value:
https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
Use macosx-g++ target for ANN on Mac.
Formatting improvements.
Merge pull request #4 from synthicity/make-improvements

ANN build improvements
Merge branch 'master' into remove-sparse-hash

Conflicts:
	Makefile