add option to find zlib in custom path

Some systems do not provide zlib development headers and libraries in
default search path of the compiler.  For these systems we should allow
specifying the location by --with-zlib=PATH or by setting ZLIB_PATH in
the makefile.

Signed-off-by: Robert Schiele <rschiele@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -373,7 +373,7 @@ BUILTIN_OBJS = \
 	builtin-pack-refs.o
 
 GITLIBS = $(LIB_FILE) $(XDIFF_LIB)
-EXTLIBS = -lz
+EXTLIBS =
 
 #
 # Platform specific tweaks
@@ -518,6 +518,12 @@ ifndef NO_CURL
 	endif
 endif
 
+ifdef ZLIB_PATH
+	BASIC_CFLAGS += -I$(ZLIB_PATH)/include
+	EXTLIBS += -L$(ZLIB_PATH)/lib $(CC_LD_DYNPATH)$(ZLIB_PATH)/lib
+endif
+EXTLIBS += -lz
+
 ifndef NO_OPENSSL
 	OPENSSL_LIBSSL = -lssl
 	ifdef OPENSSLDIR

configure.ac

@@ -75,6 +75,9 @@ GIT_ARG_SET_PATH(shell)
 # Define PERL_PATH to provide path to Perl.
 GIT_ARG_SET_PATH(perl)
 #
+# Define ZLIB_PATH to provide path to zlib.
+GIT_ARG_SET_PATH(zlib)
+#
 # Declare the with-tcltk/without-tcltk options.
 AC_ARG_WITH(tcltk,
 AS_HELP_STRING([--with-tcltk],[use Tcl/Tk GUI (default is YES)])