fix the CMake macros to install generated files correctly
fix the CMake macros to install generated files correctly

Summary:
Fix the `thrift_generate()` macro to use the correct paths in the `install()`
rule.  Every location that calls `thrift_generate()` in the other
CMakeLists.txt files uses an absolute path for `output_path`, and a relative
path for `include_prefix`.  Because the `install()` functions used `output`
path as the destination directory, this was causing the generated files to
incorrectly be installed as `${prefix}/include/full/path/to/source/repo/...`

e.g., if you had your fbthrift checkout at `/home/user/src/fbthrift`, when you
built and installed from it, all of the generated thrift header files would be
installed under `/usr/local/include/home/user/src/fbthrift/...`

Changing this `install()` rule to use the `include_prefix` as the install
destination seems like the easiest fix.

Reviewed By: wez

Differential Revision: D7479061

fbshipit-source-id: 3492667a1287768ebcb8f0fea16ddf748791e1ff

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift