Fix warnings
dbus_service.c: In function 'lash_dbus_service_connect_handler':
dbus_service.c:98: warning: implicit declaration of function 'lash_strset'
project.c: In function 'project_unload':
project.c:1131: warning: unused variable 'next2'
project.c:1131: warning: unused variable 'node2'
save_button.c:21:1: error: "_GNU_SOURCE" redefined
<command-line>: error: this is the location of the previous definition
main.c:21:1: error: "_GNU_SOURCE" redefined
<command-line>: error: this is the location of the previous definition
synth.c:8:1: error: "_GNU_SOURCE" redefined
<command-line>: error: this is the location of the previous definition
interface.c:25:1: error: "_GNU_SOURCE" redefined
<command-line>: error: this is the location of the previous definition
lash.c:21:1: error: "_GNU_SOURCE" redefined
<command-line>: error: this is the location of the previous definition