Put some quotes around autogenerated Doxygen options.

CMakeLists.txt

@@ -567,8 +567,8 @@ IF(DOXYGEN_FOUND)
         COPYONLY
     )
     FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "\n\n# Below auto-generated by cmake...\n\n")
-    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "PROJECT_NUMBER = ${PHYSFS_VERSION}\n")
-    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "OUTPUT_DIRECTORY = ${CMAKE_CURRENT_BINARY_DIR}/docs\n")
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "PROJECT_NUMBER = \"${PHYSFS_VERSION}\"\n")
+    FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "OUTPUT_DIRECTORY = \"${CMAKE_CURRENT_BINARY_DIR}/docs\"\n")
     FILE(APPEND "${PHYSFS_OUTPUT_DOXYFILE}" "\n# End auto-generated section.\n\n")
 
     ADD_CUSTOM_TARGET(