Framework: stop guessing build parameters

Either we can figure them out at runtime or they were
passed by the build system previously.

Out of band builds are no longer supported, but can be
fixed by specifying the relevant variables like the build
would.

Mk/defaults.mk

@@ -51,6 +51,13 @@ PLUGIN_FLAVOUR?=	Base
 .endif
 .endif
 
+VERSIONBIN=	${LOCALBASE}/sbin/opnsense-version
+
+.if exists(${VERSIONBIN})
+_PLUGIN_ABI!=	${VERSIONBIN} -a
+PLUGIN_ABI?=	${_PLUGIN_ABI}
+.endif
+
 PHPBIN=		${LOCALBASE}/bin/php
 
 .if exists(${PHPBIN})
@@ -65,9 +72,11 @@ _PLUGIN_PYTHON!=${PYTHONLINK} -V
 PLUGIN_PYTHON?=	${_PLUGIN_PYTHON:[2]:S/./ /g:[1..2]:tW:S/ //}
 .endif
 
-PLUGIN_ABI?=	21.1
-PLUGIN_PHP?=	74
-PLUGIN_PYTHON?=	37
+.for REPLACEMENT in ABI PHP PYTHON
+. if empty(PLUGIN_${REPLACEMENT})
+.  error Cannot build without PLUGIN_${REPLACEMENT} set
+. endif
+.endfor
 
 REPLACEMENTS=	PLUGIN_ABI \
 		PLUGIN_ARCH \