cmake: Allow user to select bundled zlib

Under some circumstances the installed / system version of zlib may not
be desirable due to being too old or buggy.  This patch adds the option
`USE_BUNDLED_ZLIB` that will cause the bundled version of zlib to be
used.

We may also want to add similar functionality to allow the user to
select other bundled 3rd-party dependencies instead of using the system
versions.

/cc @pks-t @ethomson

CMakeLists.txt

@@ -55,6 +55,7 @@ OPTION( ENABLE_WERROR			"Enable compilation with -Werror"		OFF )
 IF (UNIX AND NOT APPLE)
 	OPTION( ENABLE_REPRODUCIBLE_BUILDS	"Enable reproducible builds" 			OFF )
 ENDIF()
+OPTION( USE_BUNDLED_ZLIB    "Use the bundled version of zlib"       OFF )
 
 IF(MSVC)
 	# This option is only available when building with MSVC. By default, libgit2

src/CMakeLists.txt

@@ -255,23 +255,27 @@ ELSE()
 ENDIF()
 
 # Optional external dependency: zlib
-FIND_PACKAGE(ZLIB)
-IF (ZLIB_FOUND)
-	LIST(APPEND LIBGIT2_INCLUDES ${ZLIB_INCLUDE_DIRS})
-	LIST(APPEND LIBGIT2_LIBS ${ZLIB_LIBRARIES})
-	IF(APPLE OR CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
-		LIST(APPEND LIBGIT2_LIBS "z")
-		LIST(APPEND LIBGIT2_PC_LIBS "-lz")
+IF(NOT USE_BUNDLED_ZLIB)
+	FIND_PACKAGE(ZLIB)
+	IF(ZLIB_FOUND)
+		LIST(APPEND LIBGIT2_INCLUDES ${ZLIB_INCLUDE_DIRS})
+		LIST(APPEND LIBGIT2_LIBS ${ZLIB_LIBRARIES})
+		IF(APPLE OR CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
+			LIST(APPEND LIBGIT2_LIBS "z")
+			LIST(APPEND LIBGIT2_PC_LIBS "-lz")
+		ELSE()
+			SET(LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES} zlib")
+		ENDIF()
+		ADD_FEATURE_INFO(zlib ON "using system zlib")
 	ELSE()
-		SET(LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES} zlib")
+		MESSAGE(STATUS "zlib was not found; using bundled 3rd-party sources." )
 	ENDIF()
-	ADD_FEATURE_INFO(zlib ON "Zlib support")
-ELSE()
-	MESSAGE(STATUS "zlib was not found; using bundled 3rd-party sources." )
+ENDIF()
+IF(USE_BUNDLED_ZLIB OR NOT ZLIB_FOUND)
 	ADD_SUBDIRECTORY("${libgit2_SOURCE_DIR}/deps/zlib" "${libgit2_BINARY_DIR}/deps/zlib")
 	LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/zlib")
 	LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:zlib>)
-	ADD_FEATURE_INFO(zlib ON "Zlib support (bundled)")
+	ADD_FEATURE_INFO(zlib ON "using bundled zlib")
 ENDIF()
 
 # Optional external dependency: libssh2