Revert "fixed broken pkgconfig when liburcu is not present"

This reverts commit 2ce6a7eb.

configure.ac

@@ -259,7 +259,7 @@ AS_IF([test "$enable_daemon" = "yes"],[
 
 AS_IF([test "$with_urcu" != "no"], [
   PKG_CHECK_MODULES([liburcu], liburcu, [
-    liburcu_PKGCONFIG=",liburcu >= 0.4.0"
+    liburcu_PKGCONFIG="liburcu >= 0.4.0"
     AC_SUBST([liburcu_PKGCONFIG])
     with_urcu=yes
   ],[

src/libknot.pc.in

@@ -7,7 +7,7 @@ Name: libknot
 Description: Knot DNS library
 URL: https://www.knot-dns.cz
 Version: @PACKAGE_VERSION@
-Requires.private: libdnssec = @PACKAGE_VERSION@ @liburcu_PKGCONFIG@
+Requires.private: libdnssec = @PACKAGE_VERSION@, @liburcu_PKGCONFIG@
 Libs: -L${libdir} -lknot
 Libs.private: -ldnssec @lmdb_LIBS@
 Cflags: -I${includedir} @lmdb_CFLAGS@