Move feature-test macros to compiler flags

Feature test macros must be defined before any system header is
included. Using add_project_arguments() guarantees this, and avoids
redefinition warnings when they are already defined independently:

    In file included from ../app/src/compat.h:4,
                     from ../app/src/compat.c:1:
    app/config.h:38:9: warning: ‘_GNU_SOURCE’ redefined
       38 | #define _GNU_SOURCE
          |         ^~~~~~~~~~~

PR #6782 <https://github.com/Genymobile/scrcpy/pull/6782>