Introduce xbps_xasprintf() and use it where required.

--HG--
extra : convert_revision : 324a92a4824476059e0993d00c2b925c732a79f8

vars.mk

@@ -9,6 +9,7 @@ INSTALL_STRIPPED ?= -s
 
 LDFLAGS += -L$(TOPDIR)/lib -L$(PREFIX)/lib -lxbps
 CPPFLAGS += -I$(TOPDIR)/include -D_BSD_SOURCE -D_XOPEN_SOURCE=600
+CPPFLAGS += -D_GNU_SOURCE
 WARNFLAGS ?= -pedantic -std=c99 -Wall -Wextra -Werror -Wshadow -Wformat=2
 WARNFLAGS += -Wmissing-declarations -Wcomment -Wunused-macros -Wendif-labels
 CFLAGS += $(WARNFLAGS) -O2 -fPIC -DPIC