Makefile fixes for systems with $CXX other than g++.

- Makefile: Use $(CXX) for compiling C++ files,
  don't override the environment's value of $CXX

- build_detect_platform: use $CXX instead of g++.

Based on bug report from Theo Schlossnagle:
http://code.google.com/p/leveldb/issues/detail?id=46

(Sync with uptream at 25807040.)
2 files changed