Fix broken macro checking in the libc++ Makefile, introduced in r241909.
This caused -std=c++0x not to be passed to the build by default.

Pointy hat to: dim
MFC after: 3 days

Allow building libc++ with any custom -std=xxx flag. Note that using
-std=gnu++98 will give a lot of warnings with clang, so please use
-std=c++0x or higher, or use NO_WERROR.

MFC after: 1 week

Add libcxxrt's objects (prefixing them with cxxrt_ to avoid collisions)
to libc++.a and libc++_p.a, to make static linking of C++ executables
with libc++ easier. This is similar to the approach used

Add libcxxrt's objects (prefixing them with cxxrt_ to avoid collisions)
to libc++.a and libc++_p.a, to make static linking of C++ executables
with libc++ easier. This is similar to the approach used in libstdc++.

MFC after: 2 weeks

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