Build CompatibilitySpan as STATIC for build tools

In some cases, like when building the Swift toolchain build tools,
`SWIFT_STDLIB_LIBRARY_BUILD_TYPES` is not defined. This causes a
configuration failure when the `CompatibilitySpan` library is being
added.

This commit sets a default value of `STATIC` to work around this
problem.

Author: Steelskin

stdlib/toolchain/CompatibilitySpan/CMakeLists.txt

@@ -2,7 +2,14 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
 
   set(library_name "swiftCompatibilitySpan")
 
-  add_swift_target_library("${library_name}" ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
+  if(SWIFT_STDLIB_LIBRARY_BUILD_TYPES)
+    set(COMPATIBILITY_SPAN_BUILD_TYPES "${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}" ${COMPATIBILITY_SPAN_BUILD_TYPES} IS_STDLIB
     FakeStdlib.swift
     ../../public/core/Span/RawSpan.swift
     ../../public/core/Span/Span.swift
@@ -31,4 +38,4 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}" -Xlinker -not_for_dyld_shared_cache
 
     BACK_DEPLOYMENT_LIBRARY 6.2)
-endif()
+endif()