Log message:
lang: Remove gcc3.

Log message:
lang: align variable assignments

pkglint -Wall -F --only aligned --only indent -r

No manual corrections.

Log message:
Fix indentation in buildlink3.mk files.

The actual fix as been done by "pkglint -F */*/buildlink3.mk", and was
reviewed manually.

There are some .include lines that still are indented with zero spaces
although the surrounding .if is indented. This is existing practice.

Log message:
Replace all references to _GCC_REQD with the appropriate logical value for
each GCC version.  Using the variable causes impossible version constraints
when a specific GCC is depended upon but the user is using something newer,
as _GCC_REQD will be set to the higher value.

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

Log message:
Set BUILDLINK_ABI_DEPENDS correctly (with +=, not ?=)
It turns out there were a lot of these.

Log message:
Remove @dirrm entries from PLISTs

Log message:
Simply and speed up buildlink3.mk files and processing.
This changes the buildlink3.mk files to use an include guard for the
recursive include. The use of BUILDLINK_DEPTH, BUILDLINK_DEPENDS,
BUILDLINK_PACKAGES and BUILDLINK_ORDER is handled by a single new
variable BUILDLINK_TREE. Each buildlink3.mk file adds a pair of
enter/exit marker, which can be used to reconstruct the tree and
to determine first level includes. Avoiding := for large variables
(BUILDLINK_ORDER) speeds up parse time as += has linear complexity.
The include guard reduces system time by avoiding reading files over and
over again. For complex packages this reduces both %user and %sys time to
half of the former time.