b2/: move variable-length strings to vstring.[ch], for sharing

b2/Makefile

@@ -14,7 +14,7 @@ SHELL = /bin/bash
 CFLAGS = -g -Wall $(shell pkg-config --cflags glib-2.0)
 SLOPPY = -Wno-unused -Wno-implicit-function-declaration
 OBJS = boom.o chr.o comp.o db.o dump.o eval.o param.o subex.o subst.o util.o \
-       lex.yy.o y.tab.o
+       vstring.o lex.yy.o y.tab.o
 LDLIBS = -lfl $(shell pkg-config --libs glib-2.0)
 
 YACC = bison -y