cmake: Allow version numbers to be cache variables.

This change allows them to be overridden from the command line yet still
keep default values.

Author: Doug Coleman

cmake/modules/StandaloneOverlay.cmake

@@ -15,10 +15,10 @@ set(BUILD_STANDALONE TRUE)
 set(SWIFT_STDLIB_BUILD_TYPE "Release")
 set(SWIFT_SDK_OVERLAY_LIBRARY_BUILD_TYPES "SHARED")
 set(SWIFT_INSTALL_COMPONENTS "sdk-overlay" CACHE STRING "")
-set(SWIFT_DARWIN_DEPLOYMENT_VERSION_OSX "10.9")
-set(SWIFT_DARWIN_DEPLOYMENT_VERSION_IOS "7.0")
-set(SWIFT_DARWIN_DEPLOYMENT_VERSION_TVOS "9.0")
-set(SWIFT_DARWIN_DEPLOYMENT_VERSION_WATCHOS "2.0")
+set(SWIFT_DARWIN_DEPLOYMENT_VERSION_OSX "10.9" CACHE STRING "")
+set(SWIFT_DARWIN_DEPLOYMENT_VERSION_IOS "7.0" CACHE STRING "")
+set(SWIFT_DARWIN_DEPLOYMENT_VERSION_TVOS "9.0" CACHE STRING "")
+set(SWIFT_DARWIN_DEPLOYMENT_VERSION_WATCHOS "2.0" CACHE STRING "")
 
 set(SWIFT_SOURCE_DIR "${SWIFT_SOURCE_ROOT}/src/swift" CACHE PATH "")
 set(SWIFT_PATH_TO_LLVM_SOURCE "${SWIFT_SOURCE_ROOT}/src/llvm" CACHE PATH "")