OS X build with premake4 -- dtoa.make needs edits

[dcurrie]

After running build/premake.sh using premake-4.3 installed by Brew, a make config=release32 or make config=release64 in the gmake directory fails. This is on OS X 10.11.2 with

e$ cc -v
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.2.0
Thread model: posix

Removing the -Wl,-x from LDFLAGS in the release32 and release64 sections of dtoa.make seems to fix it.

The -L/usr/lib64 and -L/usr/lib32 in those sections are also not needed on OS X and produce warnings.