Log message:
*: Remove many GCC_REQD for particularly old GCCs

In many cases the reasoning is poorly documented, or the reasoning
for setting GCC_REQD is to work around a compiler bug. Practically
a c99 compiler is now required for bootstrap (in at least several
dependencies) so we can entirely rule out GCC versions before 3.0.

Log message:
x11: Replace RMD160 checksums with BLAKE2s checksums

All checksums have been double-checked against existing RMD160 and
SHA512 hashes

Unfetchable distfiles (fetched conditionally?):
./x11/py-qt4/distinfo PyQt4_gpl_mac-4.12.3.tar.gz

Log message:
x11: Remove SHA1 hashes for distfiles

Log message:
Whitespace.

Log message:
Add SHA512 digests for distfiles for x11 category

Problems found locating distfiles:
        Package modular-xorg-server: missing distfile xorg-server-1.17.4.tar.bz2
        Package py-qt4: missing distfile PyQt-mac-gpl-4.11.1.tar.gz
        Package xservers: missing distfile xservers-3.3.6.5.tar.bz2
        Package xview-clients: missing distfile xview3.2p1-X11R6.tar.gz
        Package xview-lib: missing distfile xview3.2p1-X11R6.tar.gz

Otherwise, existing SHA1 digests verified and found to be the same on
the machine holding the existing distfiles (morden).  All existing
SHA1 digests retained for now as an audit trail.

Log message:
Remove USE_X11BASE and X11PREFIX.

Log message:
Clean up libXaw handling. [1/2]

- remove LIBXAW variable. It is handled by buildlink3.mk now
- simplify patches and Makefile in packages using libXaw
- in some cases force use of Xaw3d (won't build with Xaw)
- replace some directly included of x11/Xaw3d with mk/xaw.buildlink3.mk

In next part:
- replace more includes with mk/xaw.buildlink3.mk

Log message:
Drop superfluous PKG_DESTDIR_SUPPORT, "user-destdir" is default these days.