pnginfo: extract eXIf chunk data

tools/Makefile

@@ -1,13 +1,14 @@
 SHELL = /bin/sh
 # libjq 1.6 lacks a pkg-config file, and there is no release in sight.
-CFLAGS = -g -O2 -Wall -Wextra `pkg-config --cflags $(deps)`
 # libjq 1.6 is required.
+CFLAGS = -g -O2 -Wall -Wextra `pkg-config --cflags $(deps)`
 LDLIBS = -ljq `pkg-config --libs $(deps)`
 
 deps = libpng
 targets = pnginfo jpeginfo
 
 all: $(targets)
+$(targets): info.h
 clean:
 	rm -f $(targets)