build: support multiple builds of SwiftPrivateLibcExtras

Support building SwiftPrivateLibcExtras for a number of targets simultaneously.
This will enable cross-compiling the standard library for multiple foreign
targets.

Author: compnerd

stdlib/private/SwiftPrivateLibcExtras/CMakeLists.txt

@@ -1,21 +1,16 @@
-set(swift_private_darwin_extras_module_depends
-  SwiftPrivate)
-
-if(SWIFT_HOST_VARIANT MATCHES "${SWIFT_DARWIN_VARIANTS}")
-  list(APPEND swift_private_darwin_extras_module_depends
-    Darwin)
-elseif(SWIFT_HOST_VARIANT STREQUAL "linux" OR SWIFT_HOST_VARIANT STREQUAL "freebsd")
-  list(APPEND swift_private_darwin_extras_module_depends
-    Glibc)
-endif()
-
 add_swift_library(swiftSwiftPrivateLibcExtras ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
   # This file should be listed the first.  Module name is inferred from the
   # filename.
   SwiftPrivateLibcExtras.swift
   Subprocess.c
   Subprocess.swift
 
-  SWIFT_MODULE_DEPENDS ${swift_private_darwin_extras_module_depends}
+  SWIFT_MODULE_DEPENDS SwiftPrivate
   SWIFT_COMPILE_FLAGS ${STDLIB_SIL_SERIALIZE_ALL}
+  SWIFT_MODULE_DEPENDS_OSX Darwin
+  SWIFT_MODULE_DEPENDS_IOS Darwin
+  SWIFT_MODULE_DEPENDS_TVOS Darwin
+  SWIFT_MODULE_DEPENDS_WATCHOS Darwin
+  SWIFT_MODULE_DEPENDS_LINUX Glibc
+  SWIFT_MODULE_DEPENDS_FREEBSD Glibc
   INSTALL_IN_COMPONENT stdlib-experimental)