cmake: remove redundant _GNU_SOURCE definition

As reported by @francoisk:

    `_GNU_SOURCE` is defined in the private config header but also on the
    compiler command-line for every object.

Fixes: libevent#1333

CMakeLists.txt

@@ -397,7 +397,6 @@ if (NOT DEFINED _GNU_SOURCE)
 endif()
 
 if (_GNU_SOURCE)
-    add_definitions(-D_GNU_SOURCE=1)
     set(CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
 endif()