Index: mk/cmake/VariableOption.cmake
===================================================================
--- mk/cmake/VariableOption.cmake	(revision 0)
+++ mk/cmake/VariableOption.cmake	(revision 0)
@@ -0,0 +1,58 @@
+# Copyright Nathan Phillip Brink <ohnobinki@ohnopublishing.net>
+# 
+#  This file is part of SuperTux.
+#
+#  SuperTux is free software: you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation, either version 3 of the License, or
+#  (at your option) any later version.
+#
+#  SuperTux is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with SuperTux.  If not, see <http://www.gnu.org/licenses/>.
+#
+#  - - -
+#
+#  SETIFNOTDEFINED will set the variable named by VARNAME to VALUE unless
+#  if it's already defined. If it is not set, it is set it to 
+#  VALUE
+#  When setting (or re-setting) the variable, the macro will insert it into
+#  the cmake cache with a docstring of ${VARNAME}_DOCSTRING and a variable type 
+#  of TYPE (e.g., PATH).
+#
+#  Usage example:
+#  SET(MYVAR_DOCSTRING "this variable exists")
+#
+#  IF(A)
+#    SETIFNOTDEFINED(MYVAR "default value" STRING)
+#  ELSE(A)
+#    SETIFNOTDEFINED(MYVAR "default value for when A is false" STRING)
+#  ENDIF(A)
+#
+#  Necessity:
+#
+#    Even though the command-line -D option for cmake overrides CMakeLists.txt's 
+#  attempt to set values in the cache, the SET() command will set variables locally
+#  while the script is executing. Thus, to recognize a user's option, the script must
+#  check if the variable is defined or not first. 
+#
+#    Because there are three variables pertaining to file installation and three 
+#  different defaults for different platforms, placing an IF() statement around each SET
+#  statement is impractical
+
+MACRO(SETIFNOTDEFINED VARNAME VALUE TYPE)
+  IF(NOT DEFINED ${VARNAME})
+    #set the default value and store in cache
+    SET(${VARNAME} ${VALUE} CACHE ${TYPE} ${${VARNAME}_DOCSTRING})
+    #I cannot use the PARENT_SCOPE and CACHE option at the same time...
+    SET(${VARNAME} ${VALUE} PARENT_SCOPE)
+  ELSE(NOT DEFINED ${VARNAME})
+    # set the variable into the cache; we use force because we want to add the 
+    # docstring even if the user specifies the variable
+    SET(${VARNAME} ${${VARNAME}} CACHE ${TYPE} ${${VARNAME}_DOCSTRING} FORCE)
+  ENDIF(NOT DEFINED ${VARNAME})
+ENDMACRO(SETIFNOTDEFINED)
\ No newline at end of file
Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt	(revision 5883)
+++ CMakeLists.txt	(working copy)
@@ -349,11 +349,17 @@
 
 ## Install stuff
 
+SET(INSTALL_SUBDIR_BIN_DOCSTRING "Directory to install binary files to")
+SET(INSTALL_SUBDIR_SHARE_DOCSTRING "Directory to install data files to")
+SET(INSTALL_SUBDIR_DOC_DOCSTRING "Directory to install doc files to")
+
+INCLUDE(VariableOption)
+
 IF(WIN32 AND NOT UNIX)
 
-  SET(INSTALL_SUBDIR_BIN ".")
-  SET(INSTALL_SUBDIR_SHARE "data/")
-  SET(INSTALL_SUBDIR_DOC ".")
+  SETIFNOTDEFINED(INSTALL_SUBDIR_BIN "." STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_SHARE "data/" STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_DOC "." STRING)
 
  INSTALL(FILES ${SUPERTUX_SOURCE_DIR}/SDL.dll
                ${SUPERTUX_SOURCE_DIR}/SDL_image.dll
@@ -369,19 +375,19 @@
 ELSE(WIN32 AND NOT UNIX)
 IF(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 
-  SET(INSTALL_SUBDIR_BIN "SuperTux.app/Contents/MacOS/")
-  SET(INSTALL_SUBDIR_SHARE "SuperTux.app/Contents/Resources/data/")
-  SET(INSTALL_SUBDIR_DOC "SuperTux.app/Contents/Resources/")
-
+  SETIFNOTDEFINED(INSTALL_SUBDIR_BIN "SuperTux.app/Contents/MacOS/" STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_SHARE "SuperTux.app/Contents/Resources/data/" STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_DOC "SuperTux.app/Contents/Resources/" STRING)
+  
   INSTALL(FILES ${SUPERTUX_SOURCE_DIR}/tools/darwin/info.plist DESTINATION "SuperTux.app/Contents/")
 
   INSTALL(FILES ${SUPERTUX_SOURCE_DIR}/data/images/engine/icons/supertux.png ${SUPERTUX_SOURCE_DIR}/data/images/engine/icons/supertux.icns DESTINATION "SuperTux.app/Contents/Resources/")
 
 ELSE(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 
-  SET(INSTALL_SUBDIR_BIN "games/")
-  SET(INSTALL_SUBDIR_SHARE "share/games/supertux2/")
-  SET(INSTALL_SUBDIR_DOC "share/doc/supertux2/")
+  SETIFNOTDEFINED(INSTALL_SUBDIR_BIN "games" STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_SHARE "share/games/supertux2" STRING)
+  SETIFNOTDEFINED(INSTALL_SUBDIR_DOC "share/doc/supertux2" STRING)
 
   INSTALL(FILES ${SUPERTUX_SOURCE_DIR}/supertux2.desktop DESTINATION "share/applications")