CMake: Add benchmarks

Put each benchmark application in a sub-directory instead of specifying all in one
file, because cotire cannot create PCH targets for the same file twice (bench.cpp).

build/cmake/main.cmake

@@ -32,6 +32,10 @@ if(wxBUILD_DEMOS)
     add_subdirectory(build/cmake/demos demos)
 endif()
 
+if(wxBUILD_BENCHMARKS)
+    add_subdirectory(build/cmake/benchmarks benchmarks)
+endif()
+
 if(NOT wxBUILD_CUSTOM_SETUP_HEADER_PATH)
     # Write setup.h after all variables are available
     include(build/cmake/setup.cmake)