(folly) make check on OSX Yosemite (10.10)

(folly) make check on OSX Yosemite (10.10)

Summary:
This switches from gcc-4.9 provided by Homebrew to the default g++ (clang) from Yosemite.

$ g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix

Absolute path for double-conversion, so make in `test/` works.

gtest-1.7.0 (Using 1.6.0 causes compilation errors in Yosemite)

Various portability fixes.

Enhancements/fixes to `configure.ac` and associated source code for better portability.

Test Plan:
It builds and runs (yay)
Two tests fail, not sure whether that's specific to osx. Will investigate and make any fixes in a follow-up diff

Reviewed By: pgriess@fb.com

Subscribers: trunkagent, folly-diffs@, exa

FB internal diff: D1768669

Tasks: 5936226

Signature: t1:1768669:1420728580:dd6919c21e0edf30788e523b16e3a5b923e2d7f0