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/benchmarks/CMakeLists.txt

@@ -0,0 +1,12 @@
+#############################################################################
+# Name:        build/cmake/benchmarks/CMakeLists.txt
+# Purpose:     CMake file for benchmarks
+# Author:      Maarten Bent
+# Created:     2021-02-07
+# Copyright:   (c) 2021 wxWidgets development team
+# Licence:     wxWindows licence
+#############################################################################
+
+add_subdirectory(bench)
+add_subdirectory(bench_graphics)
+add_subdirectory(bench_gui)