makepkg: allow make-style environment var overrides

This allows for VAR=value and VAR+=value variable declarations.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>

doc/makepkg.8.txt

@@ -11,7 +11,7 @@ makepkg - package build utility
 
 Synopsis
 --------
-'makepkg' [options]
+'makepkg' [options] [ENVVAR=value[ENVVAR+=value] ...
 
 
 Description

scripts/makepkg.sh.in

@@ -2624,6 +2624,16 @@ while true; do
 	shift
 done
 
+# attempt to consume any extra argv as environment variables. this supports
+# overriding (e.g. CC=clang) as well as overriding (e.g. CFLAGS+=' -g').
+extra_environment=()
+while [[ $1 ]]; do
+	if [[ $1 = [_[:alpha:]]*([[:alnum:]_])?(+)=* ]]; then
+		extra_environment+=("$1")
+	fi
+	shift
+done
+
 # setup signal traps
 trap 'clean_up' 0
 for signal in TERM HUP QUIT; do
@@ -2706,6 +2716,11 @@ if [[ ! -w $BUILDDIR ]]; then
 	exit 1
 fi
 
+# override settings from extra variables on commandline, if any
+if (( ${#extra_environment[*]} )); then
+	export "${extra_environment[@]}"
+fi
+
 PKGDEST=${_PKGDEST:-$PKGDEST}
 PKGDEST=${PKGDEST:-$startdir} #default to $startdir if undefined
 if (( ! (NOBUILD || GENINTEG) )) && [[ ! -w $PKGDEST ]]; then