Wrap interface include directories with BUILD_INTERFACE generator exp…

…ression (google#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

CMakeLists.txt

@@ -192,7 +192,7 @@ foreach(lib IN LISTS BROTLI_SHARED_LIBS BROTLI_STATIC_LIBS)
   if(NOT BROTLI_EMSCRIPTEN)
     set_target_properties(${lib} PROPERTIES POSITION_INDEPENDENT_CODE TRUE)
   endif()
-  set_property(TARGET ${lib} APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES "${BROTLI_INCLUDE_DIRS}")
+  set_property(TARGET ${lib} APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES "$<BUILD_INTERFACE:${BROTLI_INCLUDE_DIRS}>")
 endforeach()
 
 if(NOT BROTLI_EMSCRIPTEN)