build: dereference variable as appropriate

Certain versions of CMake behave differently with variable expansion.  This may
be evaluated incorrectly in some versions.  Dereference the value explicitly.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -2370,7 +2370,7 @@ function(_add_swift_executable_single name)
   # NOTE(compnerd) use the C linker language to invoke `clang` rather than
   # `clang++` as we explicitly link against the C++ runtime.  We were previously
   # actually passing `-nostdlib++` to avoid the C++ runtime linkage.
-  if(SWIFTEXE_SINGLE_SDK STREQUAL ANDROID)
+  if(${SWIFTEXE_SINGLE_SDK} STREQUAL ANDROID)
     set_property(TARGET "${name}" PROPERTY
       LINKER_LANGUAGE "C")
   else()