GraphBLAS: Check for ability to link with libdl.

On some platforms (like NetBSD), `dlopen` and similar functions are not
in a library. They can be used in any dynamically linked program instead:
https://man.netbsd.org/dlopen.3

Use a feature test during configuration to check whether linking to libdl
is necessary (or possible).

GraphBLAS/CMakeLists.txt

@@ -409,7 +409,9 @@ if ( NOT NO_LIBM )
 endif ( )
 
 # libdl
-if ( NOT WIN32 )
+include ( CheckLibraryExists )
+check_library_exists ( "dl" "dlopen" "${CMAKE_LIBRARY_PATH}" HAVE_DL )
+if ( HAVE_DL AND NOT WIN32 )
     if ( BUILD_SHARED_LIBS )
         target_link_libraries ( GraphBLAS PRIVATE dl )
     endif ( )