GraphBLAS: Use library as provided by CMake for dlopen and Co

Most UNIX-like systems provide `dlopen` and similar functions in a
library named libdl.

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

Other platforms (e.g., IBM AIX) provide these functions in a differently
named library (e.g., libld).

Use the name of the library containing `dlopen` and similar functions as
provided by CMake.

GraphBLAS/CMakeLists.txt

@@ -409,13 +409,13 @@ if ( NOT NO_LIBM )
 endif ( )
 
 # libdl
-if ( NOT WIN32 )
+if ( NOT "${CMAKE_DL_LIBS}" STREQUAL "" )
     if ( BUILD_SHARED_LIBS )
-        target_link_libraries ( GraphBLAS PRIVATE dl )
+        target_link_libraries ( GraphBLAS PRIVATE ${CMAKE_DL_LIBS} )
     endif ( )
     if ( BUILD_STATIC_LIBS )
-        list ( APPEND GRAPHBLAS_STATIC_LIBS "dl" )
-        target_link_libraries ( GraphBLAS_static PUBLIC dl )
+        list ( APPEND GRAPHBLAS_STATIC_LIBS ${CMAKE_DL_LIBS} )
+        target_link_libraries ( GraphBLAS_static PUBLIC ${CMAKE_DL_LIBS} )
     endif ( )
 endif ( )