Mk/bsd.port.mk: Allow category_port_VARS= in make.conf for arbitrary …

…per-port overrides.

Currently the only way to specify make.conf variables (other than port
options which have their own mechanism) in a per-port fashion is to use
conditionals on .CURDIR, which is fragile since it tends to involve
assumptions about where the ports tree is mounted.

Instead, allow category_portname_VARS= to be set in make.conf to provide
arbitrary assignments when building a specified port. For example one
might use

    devel_llvm10_VARS= MAKE_JOBS_NUMBER=2

or

    converters_lua-iconv_VARS= TRYBROKEN=yes

This is intended to be consistent with the existing
category_portname_SET= variables for port options, and uses the same
syntax for values as option_VARS= in port makefiles, but without the
case-folding behavior since that makes it impossible to handle
mixed-case or lowercase variable names.

Reviewed by:	koobs, mat, adamw, mandree, 0mp
Approved by:	portmgr (bapt)
Differential Revision:	https://reviews.freebsd.org/D24324
VAR+=val can be used to append to a variable, and VAR@ to unset one entirely.

Mk/bsd.port.mk

@@ -1283,6 +1283,23 @@ _PORTDIRNAME=	${.CURDIR:T}
 PORTDIRNAME?=	${_PORTDIRNAME}
 PKGORIGIN?=		${PKGCATEGORY}/${PORTDIRNAME}
 
+# Now that PKGORIGIN is set, look for origin-specific variables.
+# These are typically set in a make.conf, in the form:
+#
+# category_portname_VARS= varname=value othervar+=value novar@
+#
+# e.g.  devel_llvm10_VARS= MAKE_JOBS_NUMBER=2
+
+.    for var in ${${PKGORIGIN:S/\//_/}_VARS:C/=.*//:O:u}
+.      if ${var:M*@}
+.  undef ${var:C/.$//}
+.      elif ${var:M*+}
+${var:C/.$//}+=	${${PKGORIGIN:S/\//_/}_VARS:M${var}=*:C/[^+]*\+=//:C/^"(.*)"$$/\1/}
+.      else
+${var}=			${${PKGORIGIN:S/\//_/}_VARS:M${var}=*:C/[^=]*=//:C/^"(.*)"$$/\1/}
+.      endif
+.    endfor
+
 # where 'make config' records user configuration options
 PORT_DBDIR?=	/var/db/ports