Support for firebreath as a subproject

When you want to integrate FireBreath in your own project you can use the following (sample) in your CMakeLists.txt:

```cmake
set(FB_USE_AS_SUBPROJECT 1)

set(FB_PROJECTS_DIR  ${CMAKE_CURRENT_SOURCE_DIR}/web_plugin/xyz)
set(PROJECT_PATH "${FB_PROJECTS_DIR}")

set(FIREBREATH_ROOT path_to_firebreath CACHE PATH "FireBreath root" FORCE)
add_subdirectory(${FIREBREATH_ROOT} ${PROJECT_ROOT}/build/plugin )

```

Sorry, i've tested only with Windows.