[offload] Include CheckCXXCompilerFlag in CMake

Include CheckCXXCompilerFlag before it is used in the top-level CMake
file.  This fixes standalone builds, but it is also cleaner than
assuming that some previous CMake file will include it for us.

Author: mgorny

offload/CMakeLists.txt

@@ -129,6 +129,7 @@ include(LibomptargetGetDependencies)
 # Set up testing infrastructure.
 include(OpenMPTesting)
 
+include(CheckCXXCompilerFlag)
 check_cxx_compiler_flag(-Werror=global-constructors OFFLOAD_HAVE_WERROR_CTOR)
 
 # LLVM source tree is required at build time for libomptarget