Cmake build: redundant _GNU_SOURCE definition

[francoisk]

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

https://github.com/libevent/libevent/blob/b19af675c7601a7867f26c33072cda7ea125adb2/CMakeLists.txt#L399-L402

https://github.com/libevent/libevent/blob/b19af675c7601a7867f26c33072cda7ea125adb2/evconfig-private.h.cmake#L8-L10

[azat]

Refs #460