Move all _GNU_SOURCE defination to the file head

[Seas0]

Accroding to man 7 feature_test_macros' guide line here:
NOTE: In order to be effective, a feature test macro must be defined before including any header files. This can be done either in the compilation command
(cc -DMACRO=value) or by defining the macro within the source code before including any headers. The requirement that the macro must be defined before in‐
cluding any header file exists because header files may freely include one another.