fixed ucred compile warnings

modified:   src/accelerometer/Makefile.am
modified:   src/ata_id/Makefile.am
modified:   src/cdrom_id/Makefile.am
modified:   src/collect/Makefile.am
modified:   src/scsi_id/Makefile.am
Adding -D_GNU_SOURCE flag incorporates ucred def in header files
so that 'ucred undeclared' warning do not occur during compilation
against glibc.