wavemon: unset CFLAGS to fix a build error.

The error:

    scan_scr.o: In function `display_aplist':
    scan_scr.c:(.text+0x5a6): undefined reference to `pthread_mutex_trylock'
    scan_scr.o: In function `scr_aplst_init':
    scan_scr.c:(.text+0xcea): undefined reference to `pthread_create'
    scan_scr.o: In function `scr_aplst_fini':
    scan_scr.c:(.text+0xddc): undefined reference to `pthread_cancel'
    iw_scan.o: In function `do_scan':
    iw_scan.c:(.text+0xf15): undefined reference to `pthread_detach'
    collect2: error: ld returned 1 exit status
    <builtin>: recipe for target 'wavemon' failed