`add_fbthrift_library` should add include path for prefix

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Commit 64e0e3f introduced includes from fb303 to fbthrift. This currently fails if one tries to build fb303 as a system library, because the include paths passed to thrift1 does not include /usr/include which is where the Thrift annotations included by fb303_thrift live (installed by a prior fbthrift installation). Either fbthrift1 should look at the prefix by default, or the CMake custom command generated by add_fbthrift_library should include it.

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Actually, there already exists a FBTHRIFT_INCLUDE_DIR set by FBThriftConfig.cmake, so that should either be passed as a default argument or defaulted by FBThriftConfig.cmake somehow (by providing a wrapper).

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Relevant commit which fixes this for the C++ library but not for the Python library: 536ff1c