Fix CMake script for NetBSD and OpenBSD.

CMakeLists.txt

@@ -450,7 +450,12 @@ endif()
 set(COMMON_LINK_LIBRARIES)
 
 if (UNIX)
-    list(APPEND COMMON_LINK_LIBRARIES dl)      # For dlopen(), etc. Most compilers will link to this by default, but some may not.
+    # Not all platforms actually use a separate "dl" library, notably NetBSD and OpenBSD.
+    find_library(LIB_DL "dl")
+    if(LIB_DL)
+        list(APPEND COMMON_LINK_LIBRARIES dl)  # For dlopen(), etc. Most compilers will link to this by default, but some may not.
+    endif()
+    
     list(APPEND COMMON_LINK_LIBRARIES pthread) # Some compilers will not link to pthread by default so list it here just in case.
     list(APPEND COMMON_LINK_LIBRARIES m)