Link with $CXXFLAGS; disable GCC-specific autoconf check.

Using CXXFLAGS with LDFLAGS is needed when compiling with ```-stdlib=libc++``` (necessary if building as C++11 with clang). Otherwise, it will try to link against libstdc++ and fails due to differences in the implementation of some STL functions. ```xcodebuild -configuration Release_C++11``` already does this.

```if test "$GCC"```... currently prevents ```-fno-exceptions``` and ```-rdynamic``` from being used when CXX is not g++. Also, I figured the ```-fno-optimize-sibling-calls``` bit below probably is no longer necessary. Unless anyone is still building with GCC 3.3.3 on NetBSD 3.0?