[build] Allow building against a zlib in a non-standard location

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 DOXYGEN		:= doxygen
 BINUTILS_DIR	:= /usr
 BFD_DIR		:= $(BINUTILS_DIR)
+ZLIB_DIR	:= /usr
 
 ###############################################################################
 #

src/Makefile.housekeeping

 # The EFI image converter
 #
 ELF2EFI_CFLAGS	:= -I$(BINUTILS_DIR)/include -I$(BFD_DIR)/include \
-		   -idirafter include -L$(BINUTILS_DIR)/lib -L$(BFD_DIR)/lib \
+		   -I$(ZLIB_DIR)/include -idirafter include \
+		   -L$(BINUTILS_DIR)/lib -L$(BFD_DIR)/lib -L$(ZLIB_DIR)/lib \
 		   -lbfd -liberty -lz -Wl,--no-warn-search-mismatch
 
 $(ELF2EFI32) : util/elf2efi.c $(MAKEDEPS)