Fix building zchunk code if zchunk is enabled

Commit 66c99da (Change header files
to match a configured ABI regarding a zchunk support) accidentally
removed a definition of WITH_ZCHUNK C preprocessor symbol. That
symbol controls including zchunk support into the library's code.

As a result, librepo library was always missing a zchunk support and
ci-dnf-stack "dnf/zchunk.feature:141 using mirror wihtout ranges
supports and zchunk results in only two GET requests for primary (the
first try is with range specified)" always failed.

This fix returns back the macro into CFLAGS.

Author: ppisar

CMakeLists.txt

@@ -71,6 +71,8 @@ ENDIF (USE_RUN_GNUPG_USER_SOCKET)
 
 IF (WITH_ZCHUNK)
     PKG_CHECK_MODULES(ZCHUNKLIB zck>=0.9.11 REQUIRED)
+    SET (CMAKE_C_FLAGS          "${CMAKE_C_FLAGS} -DWITH_ZCHUNK")
+    SET (CMAKE_C_FLAGS_DEBUG    "${CMAKE_C_FLAGS_DEBUG} -DWITH_ZCHUNK")
     SET (LIBREPO_ZCHUNK_ENABLED "1")
 ELSE (WITH_ZCHUNK)
     SET (LIBREPO_ZCHUNK_ENABLED "0")