Only configure libtool for libraries that exist

cmake/Modules/OpmLibMain.cmake

@@ -225,7 +225,9 @@ opm_doc (${project} ${doxy_dir})
 # provide compatibility with using this build in dunecontrol
 include (DuneCompat)
 include (LibtoolArchives)
-configure_la (${project} ${${project}_TARGET} ${project}_LIBTOOL_ARCHIVE)
+if (${project}_TARGET)
+	configure_la (${project} ${${project}_TARGET} ${project}_LIBTOOL_ARCHIVE)
+endif ()
 
 ### clean in-source builds ###
 include (OpmDistClean)