ENH: Adding CMAKE_DEPENDENT_OPTION macro.

Modules/CMakeDependentOption.cmake

@@ -0,0 +1,37 @@
+# - Macro to provide an option dependent on other options.
+# This macro presents an option to the user only if a set of other
+# conditions are true.  When the option is not presented a default
+# value is used, but any value set by the user is preserved for when
+# the option is presented again.
+# Example invocation:
+#  CMAKE_DEPENDENT_OPTION(USE_FOO "Use Foo" ON
+#                         "USE_BAR;NOT USE_ZOT" OFF)
+# If USE_BAR is true and USE_ZOT is false, this provides an option called
+# USE_FOO that defaults to ON.  Otherwise, it sets USE_FOO to OFF.  If
+# the status of USE_BAR or USE_ZOT ever changes, any value for the
+# USE_FOO option is saved so that when the option is re-enabled it
+# retains its old value.
+MACRO(CMAKE_DEPENDENT_OPTION option doc default depends force)
+  IF(${option}_ISSET MATCHES "^${option}_ISSET$")
+    SET(${option}_AVAILABLE 1)
+    FOREACH(d ${depends})
+      STRING(REGEX REPLACE " +" ";" CMAKE_DEPENDENT_OPTION_DEP "${d}")
+      IF(${CMAKE_DEPENDENT_OPTION_DEP})
+      ELSE(${CMAKE_DEPENDENT_OPTION_DEP})
+        SET(${option}_AVAILABLE 0)
+      ENDIF(${CMAKE_DEPENDENT_OPTION_DEP})
+    ENDFOREACH(d)
+    IF(${option}_AVAILABLE)
+      OPTION(${option} "${doc}" "${default}")
+      SET(${option} "${${option}}" CACHE BOOL "${doc}" FORCE)
+    ELSE(${option}_AVAILABLE)
+      IF(${option} MATCHES "^${option}$")
+      ELSE(${option} MATCHES "^${option}$")
+        SET(${option} "${${option}}" CACHE INTERNAL "${doc}")
+      ENDIF(${option} MATCHES "^${option}$")
+      SET(${option} ${force})
+    ENDIF(${option}_AVAILABLE)
+  ELSE(${option}_ISSET MATCHES "^${option}_ISSET$")
+    SET(${option} "${${option}_ISSET}")
+  ENDIF(${option}_ISSET MATCHES "^${option}_ISSET$")
+ENDMACRO(CMAKE_DEPENDENT_OPTION)