Add a dependency on zck to librepo.pc if zchunk support is enabled

If librepo is built with zchunk support, zchunk's zck.h header file is
included from librepo public header files, thus applications building
against librepo needs zchunk header files available at build time.
Those who only use pkg-config for tracking dependencies, reported
build failures:

    /usr/include/librepo/downloadtarget.h:27:10: fatal error: zck.h: No such file or directory

This patch adds the dependency on zck to librepo.pc.

A drawback is that then pkg-config adds -lzck to "pkg-config --libs
lirepo" and leads to overlinking. There is no non-library kind of
dependency in pkg-config. (Ideal solution would be if pkg-config
supported those dependencies, or if zck provided a non-library
pkg-config file in addition to zck.pc.)

Fixes: #305

librepo/CMakeLists.txt

@@ -80,6 +80,7 @@ ENDIF (USE_GPGME)
 
 IF (WITH_ZCHUNK)
     TARGET_LINK_LIBRARIES(librepo ${ZCHUNKLIB_LIBRARIES})
+    SET(PKGCONF_DEPENDENCY_ZCK "zck")
 ENDIF (WITH_ZCHUNK)
 
 SET_TARGET_PROPERTIES(librepo PROPERTIES OUTPUT_NAME "repo")

librepo/librepo.pc.cmake

@@ -5,7 +5,7 @@ includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 Name: librepo
 Description: Repodata downloading library.
 Version: @VERSION@
-Requires: glib-2.0
+Requires: glib-2.0 @PKGCONF_DEPENDENCY_ZCK@
 Requires.private: libcurl openssl libxml-2.0
 Libs: -L${libdir} -lrepo
 Libs.private: -lgpgme -lgpg-error