Disable CompatibilitySpans outside of stdlib build

Author: Steelskin

stdlib/toolchain/CompatibilitySpan/CMakeLists.txt

@@ -1,15 +1,8 @@
-if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND DEFINED SWIFT_STDLIB_LIBRARY_BUILD_TYPES)
 
   set(library_name "swiftCompatibilitySpan")
 
-  if(SWIFT_STDLIB_LIBRARY_BUILD_TYPES)
-    set(COMPATIBILITY_SPAN_BUILD_TYPES "${SWIFT_STDLIB_LIBRARY_BUILD_TYPES}")
-  else()
-    # This is not set when building build tools for the toolchain build.
-    set(COMPATIBILITY_SPAN_BUILD_TYPES "STATIC")
-  endif()
-
-  add_swift_target_library("${library_name}" ${COMPATIBILITY_SPAN_BUILD_TYPES} IS_STDLIB
+  add_swift_target_library("${library_name}" ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
     FakeStdlib.swift
     ../../public/core/Span/RawSpan.swift
     ../../public/core/Span/Span.swift