Default to omitting build of (unused) UPC runtime

The Clang UPC2C source-to-source translator does not require the Clang
UPC runtime library, which builds on only a subset of the platforms
where Clang UPC2C does.  So, we should not build it (or fail to, in
some case) by default.

runtime/CMakeLists.txt

@@ -12,7 +12,7 @@ foreach (dir ${known_subdirs})
   endif()
 endforeach()
 
-set(ENABLE_CLANG_UPC_RUNTIME TRUE CACHE BOOL "enable UPC runtime library")
+set(ENABLE_CLANG_UPC_RUNTIME FALSE CACHE BOOL "enable UPC runtime library")
 if (ENABLE_CLANG_UPC_RUNTIME)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/libupc)
 endif()