cmTarget: Enable convenient include dir handling for INTERFACE_LIBRARY.

Make the CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE variable affect
INTERFACE_LIBRARY targets.

Tests/InterfaceLibrary/definetestexe.cpp

@@ -3,6 +3,18 @@
 #error Expected IFACE_DEFINE
 #endif
 
+#include "iface_header.h"
+
+#ifndef IFACE_HEADER_SRCDIR
+#error Expected IFACE_HEADER_SRCDIR
+#endif
+
+#include "iface_header_builddir.h"
+
+#ifndef IFACE_HEADER_BUILDDIR
+#error Expected IFACE_HEADER_BUILDDIR
+#endif
+
 int main(int,char**)
 {
   return 0;