stdlib: build multiple platform libraries simultaneously

Always construct the platform libraries for all the target SDKs.  This enables
us to build the right set for targets and will also enable us to cross-compile
multiple targets simultaneously.

Author: compnerd

stdlib/public/Platform/CMakeLists.txt

@@ -6,28 +6,21 @@ set(swift_platform_sources
     TiocConstants.swift
     tgmath.swift.gyb)
 
-if(SWIFT_HOST_VARIANT MATCHES "${SWIFT_DARWIN_VARIANTS}")
-  set(swift_platform_name swiftDarwin)
-  set(swift_platform_sources
-      Darwin.swift
-      ${swift_platform_sources}
-      POSIXError.swift
-      MachError.swift)
-  set(swift_platform_flags
-      SWIFT_COMPILE_FLAGS -Xfrontend -disable-objc-attr-requires-foundation-module
-      API_NOTES_NON_OVERLAY)
-else()
-  set(swift_platform_name swiftGlibc)
-  set(swift_platform_sources
-      Glibc.swift
-      ${swift_platform_sources})
-  set(swift_platform_flags
-      DEPENDS glibc_modulemap)
-endif()
+add_swift_library(swiftDarwin ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+    Darwin.swift
+    ${swift_platform_sources}
+    POSIXError.swift
+    MachError.swift
+
+    SWIFT_COMPILE_FLAGS -Xfrontend -disable-objc-attr-requires-foundation-module
+    TARGET_SDKS IOS IOS_SIMULATOR OSX TVOS TVOS_SIMULATOR WATCHOS WATCHOS_SIMULATOR
+    API_NOTES_NON_OVERLAY)
 
-add_swift_library(${swift_platform_name} ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+add_swift_library(swiftGlibc ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVERLAY
+    Glibc.swift
     ${swift_platform_sources}
-    ${swift_platform_flags})
+    TARGET_SDKS ANDROID CYGWIN FREEBSD LINUX
+    DEPENDS glibc_modulemap)
 
 set(glibc_modulemap_target_list)
 foreach(sdk ${SWIFT_SDKS})