build: avoid passing private flags from pmake

pmake introduces private flags (-J) when passing certain arguments
to it (such as -j). Filter these out before passing to gmake.

PR-URL: #1334
Reviewed-By: Fedor Indutny <fedor@indutny.com>

BSDmakefile

@@ -1,8 +1,11 @@
+# pmake might add -J (private)
+FLAGS=${.MAKEFLAGS:C/\-J ([0-9]+,?)+//W}
+
 all: .DEFAULT
 .DEFAULT:
 	@which gmake > /dev/null 2>&1 ||\
 		(echo "GMake is required for io.js to build.\
 			Install and try again" && exit 1)
-	@gmake ${.MAKEFLAGS} ${.TARGETS}
+	@gmake ${.FLAGS} ${.TARGETS}
 
 .PHONY: test