Add re2: fast & secure regular expression library

[mbunkus]

Notes:

• builds in $(SOURCE_DIR) as I don't think the custom Makefile they employ supports out-of-tree builds
• only supports static builds (as I don't think the custom Makefile & the source code supports Windows-style dynamic libraries, haven't verified, though) but doesn't enforce this; shared targets will still create static libraries
• have built successfully with x86_64-w64-mingw.static and i686-w64-mingw.static
• have run test binaries created for both architectures successfully on Windows