Add support for VPATH builds in contrib/minizip.

Instead of using relative paths directly, use paths relative to
top_srcdir and top_builddir to refer to source files and built files,
respectively.

Note that the toplevel zlib configure script still does not have any
special support for out-of-tree builds.  But now you can do

	(cd contrib/minizip && autoreconf -fis)

	mkdir -p BUILD/test
	cp *.c *.h *.in zlib.map configure zlib.3 BUILD
	cp test/*.c BUILD/test
	(cd BUILD && ./configure --shared)
	(cd BUILD && make)

	mkdir -p BUILD/contrib/minizip
	cd BUILD/contrib/minizip
	../../../contrib/minizip/configure
	make

While at it, move the include path and library path settings to
CPPFLAGS and LDFLAGS respectively instead of setting both in CFLAGS.
Thanks to Mike Frysinger for advice.

contrib/minizip/Makefile.am

 lib_LTLIBRARIES = libminizip.la
 
+zlib_top_srcdir = $(top_srcdir)/../..
+zlib_top_builddir = $(top_builddir)/../..
+
+AM_CPPFLAGS = -I$(zlib_top_srcdir)
+AM_LDFLAGS = -L$(zlib_top_builddir)
+
 libminizip_la_SOURCES = \
 	ioapi.c \
 	mztools.c \
 	unzip.c \
 	zip.c
 
-AM_CFLAGS = -I../.. -L../..
-libminizip_la_LDFLAGS = -version-info 1:0:0 -lz
+libminizip_la_LDFLAGS = $(AM_LDFLAGS) -version-info 1:0:0 -lz
 
 minizip_includedir = $(includedir)/minizip
 minizip_include_HEADERS = \