cmake: Use ${CMAKE_DL_LIBS} instead of hardcoding "dl"

Some systems ship the dlopen() family of functions in a separate "libdl"
library, some others include them in the system libc, and others may use
a different name altogether.

Instead of hardcoding "dl" as the library name, pick the needed library
from the ${CMAKE_DL_LIBS}, which has been available since CMake 3.0 at
least:

  https://cmake.org/cmake/help/v3.0/variable/CMAKE_DL_LIBS.html

CMakeLists.txt

@@ -58,7 +58,7 @@ set(WPE_INCLUDE_DIRECTORIES
 )
 
 set(WPE_LIBRARIES
-    dl
+    ${CMAKE_DL_LIBS}
     XkbCommon::libxkbcommon
 )
 
@@ -163,4 +163,4 @@ IF(BUILD_DOCS)
   ELSE()
       MESSAGE(FATAL_ERROR "Hotdoc C extension not found... can't build the documentation.")
   ENDIF()
-ENDIF()
+ENDIF()