build: mark functions with "const" or "pure" attribute, per gcc warnings

build: mark functions with "const" or "pure" attribute, per gcc warnings

Use _GL_ATTRIBUTE_PURE or _GL_ATTRIBUTE_CONST.  Mark static functions at
point of definition; extern ones at point of declaration.
* include/parted/crc32.h:
* include/parted/device.h: Likewise.
* include/parted/disk.h: Likewise.
* include/parted/exception.h: Likewise.
* include/parted/filesys.h: Likewise.
* include/parted/geom.h: Likewise.
* include/parted/natmath.h: Likewise.
* include/parted/parted.h: Likewise.
* include/parted/unit.h: Likewise.
* libparted/arch/linux.c: Likewise.
* libparted/disk.c: Likewise.
* libparted/exception.c: Likewise.
* libparted/fs/amiga/amiga.h: Likewise.
* libparted/labels/aix.c: Likewise.
* libparted/labels/bsd.c: Likewise.
* libparted/labels/dos.c: Likewise.
* libparted/labels/dvh.c: Likewise.
* libparted/labels/efi_crc32.c: Likewise.
* libparted/labels/gpt.c: Likewise.
* libparted/labels/mac.c: Likewise.
* libparted/labels/pc98.c: Likewise.
* libparted/labels/rdb.c: Likewise.
* libparted/labels/sun.c: Likewise.
* libparted/unit.c: Likewise.
* parted/ui.c: Likewise.
* parted/strlist.c: Likewise.
* parted/strlist.h: Likewise.
* libparted/tests/common.h (_implemented_disk_label): Mark as pure.
* libparted/cs/natmath.c (extended_euclid): Mark as pure.