Pass compiler flags to leveldb make

Fixes issue#2288. Includes cleanups from Luke's pull 2243.

Tested on OSX, Linux, and cross-compiling in gitian.

"We" should refactor all of the common makefile code into a makefile.common, and include it from the other makefiles.  There is an odd mix of CFLAGS / CXXFLAGS / xCXXFLAGS in our makefiles.