[gzip] Don't compile internal zlib development files when using system zlib.

`src/gzip/rules.mk` compiles the internal zlib sources even when using the
zlib development files provided by a host system.  If the internal zlib
development files are not present, FreeType fails to build from source.

This patch ensures the internal zlib development files are only
prerequisites when not using zlib development files on a host system.

* src/gzip/rules.mk (GZIP_DRV_SRCS): Define conditionally.

src/gzip/rules.mk

@@ -36,24 +36,23 @@ endif
 #
 # All source and header files get loaded by `ftgzip.c' only if SYSTEM_ZLIB
 # is not defined (regardless whether we have a `single' or a `multi' build).
-# However, it doesn't harm if we add everything as a dependency
-# unconditionally.
 #
-GZIP_DRV_SRCS := $(GZIP_DIR)/adler32.c  \
-                 $(GZIP_DIR)/crc32.c    \
-                 $(GZIP_DIR)/crc32.h    \
-                 $(GZIP_DIR)/ftzconf.h  \
-                 $(GZIP_DIR)/inffast.c  \
-                 $(GZIP_DIR)/inffast.h  \
-                 $(GZIP_DIR)/inffixed.h \
-                 $(GZIP_DIR)/inflate.c  \
-                 $(GZIP_DIR)/inflate.h  \
-                 $(GZIP_DIR)/inftrees.c \
-                 $(GZIP_DIR)/inftrees.h \
-                 $(GZIP_DIR)/zlib.h     \
-                 $(GZIP_DIR)/zutil.c    \
-                 $(GZIP_DIR)/zutil.h
-
+ifeq ($(SYSTEM_ZLIB),)
+  GZIP_DRV_SRCS := $(GZIP_DIR)/adler32.c  \
+                   $(GZIP_DIR)/crc32.c    \
+                   $(GZIP_DIR)/crc32.h    \
+                   $(GZIP_DIR)/ftzconf.h  \
+                   $(GZIP_DIR)/inffast.c  \
+                   $(GZIP_DIR)/inffast.h  \
+                   $(GZIP_DIR)/inffixed.h \
+                   $(GZIP_DIR)/inflate.c  \
+                   $(GZIP_DIR)/inflate.h  \
+                   $(GZIP_DIR)/inftrees.c \
+                   $(GZIP_DIR)/inftrees.h \
+                   $(GZIP_DIR)/zlib.h     \
+                   $(GZIP_DIR)/zutil.c    \
+                   $(GZIP_DIR)/zutil.h
+endif
 
 # gzip driver object(s)
 #