Fix library order for newer GCC linking behavior

[spgill]

Trying to build mamtool on Debian 12 fails to build with undefined references to libgc like so:

cc -Wall -pedantic -g -DUSCSI_LINUX_SCSI -DSCSI -DNO_STRLCPY -I/usr/pkg/include -c endian_utils.c
cc -Wall -pedantic -g -DUSCSI_LINUX_SCSI -DSCSI -DNO_STRLCPY -I/usr/pkg/include -c mamtool.c
cc -Wall -pedantic -g -DUSCSI_LINUX_SCSI -DSCSI -DNO_STRLCPY -I/usr/pkg/include -c uscsi_subr.c
cc -Wall -pedantic -g -DUSCSI_LINUX_SCSI -DSCSI -DNO_STRLCPY -I/usr/pkg/include -c uscsi_sense.c
cc -lgc  -o mamtool endian_utils.o uscsi_sense.o mamtool.o uscsi_subr.o
/usr/bin/ld: mamtool.o: in function `attribute_set_value':
/tmp/mamtool/src/mamtool.c:222: undefined reference to `GC_malloc'
/usr/bin/ld: mamtool.o: in function `attribute_value_to_string':
/tmp/mamtool/src/mamtool.c:243: undefined reference to `GC_malloc'
/usr/bin/ld: mamtool.o: in function `mam_write_attribute_1':
/tmp/mamtool/src/mamtool.c:419: undefined reference to `GC_malloc'
/usr/bin/ld: mamtool.o: in function `mam_read_attribute_1':
/tmp/mamtool/src/mamtool.c:442: undefined reference to `GC_malloc'
/usr/bin/ld: /tmp/mamtool/src/mamtool.c:453: undefined reference to `GC_malloc'
/usr/bin/ld: mamtool.o:/tmp/mamtool/src/mamtool.c:485: more undefined references to `GC_malloc' follow
/usr/bin/ld: mamtool.o: in function `main':
/tmp/mamtool/src/mamtool.c:714: undefined reference to `GC_init'
/usr/bin/ld: /tmp/mamtool/src/mamtool.c:719: undefined reference to `GC_strdup'
/usr/bin/ld: /tmp/mamtool/src/mamtool.c:722: undefined reference to `GC_strdup'
/usr/bin/ld: /tmp/mamtool/src/mamtool.c:735: undefined reference to `GC_strdup'

It seems that with newer versions of GCC, libraries are only linked as needed. Moving the -lgc flag to the end of the ld command fixes this build issue.

I broke the LDFLAGS into prefix and suffix flags in case flags are still wanted in that position.

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I'm not really a C developer, so please correct me if there's a better way to do this 😅