Wrap interface include directories with BUILD_INTERFACE generator expression (#966)
Wrap interface include directories with BUILD_INTERFACE generator expression (#966)

* Wrap interface include directories with BUILD_INTERFACE generator expression

When exporting a CMake target using install(TARGETS) + install(EXPORT),
CMake requires all include directories to be clean of build system
directories.

https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_INCLUDE_DIRECTORIES.html

This change also allows use of brotli as a CMake subproject and
installing + exporting it.

* Fix typo in generator expression
google/brotli,google/brotli,google/brotli,google/brotli,google/brotli,google/brotli,google/brotli,google/brotli