abuild: support for conflicts in makedepends

prefix a makedepends with ! and its treated as a conflict. for example:

makedepends="zlib-dev !gettext"

abuild

@@ -541,7 +541,10 @@ builddeps() {
 
 	# find which deps are missing
 	for i in $deps; do
-		if ! apk info -e $i; then
+		if [ "${i#\!}" != "$i" ]; then
+			apk info -e ${i#\!} \
+				&& die "Conflicting package ${i#\!} is installed."
+		elif ! apk info -e $i; then
 			if [ -z "$install_deps" ] && [ -z "$recursive" ]; then
 				die "Missing dependency $i. Use -r to autoinstall or -R to build"
 			fi