CMake: Add PHOBOS2_DIR to cache, so it would be accessible using ccmake

runtime/CMakeLists.txt

@@ -58,6 +58,7 @@ if(D_VERSION EQUAL 1)
 	file(GLOB DCRT_C ${RUNTIME_DC_DIR}/*.c)
 	set(CONFIG_NAME ${LDC_EXE})
 elseif(D_VERSION EQUAL 2)
+	set(PHOBOS2_DIR ${PROJECT_PARENT_DIR}/../${RUNTIME} CACHE PATH "phobos2 source dir")
 	set(RUNTIME_CC druntime-core)
 	set(RUNTIME_GC druntime-gc-basic)
 	set(RUNTIME_DC druntime-rt-ldc)