cmake: Convert check_cxx_source_compiles_with_flags to a function

cmake/minisketch.cmake

@@ -2,13 +2,15 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or https://opensource.org/license/mit/.
 
+include(CheckSourceCompilesAndLinks)
+
 # Check for clmul instructions support.
 if(MSVC)
-  set(CLMUL_CXXFLAGS)
+  set(CLMUL_CXXFLAGS "")
 else()
   set(CLMUL_CXXFLAGS -mpclmul)
 endif()
-check_cxx_source_compiles_with_flags("${CLMUL_CXXFLAGS}" "
+check_cxx_source_compiles_with_flags("
   #include <immintrin.h>
   #include <cstdint>
 
@@ -22,6 +24,7 @@ check_cxx_source_compiles_with_flags("${CLMUL_CXXFLAGS}" "
     return e == 0;
   }
   " HAVE_CLMUL
+  CXXFLAGS ${CLMUL_CXXFLAGS}
 )
 
 add_library(minisketch_common INTERFACE)