* Libraries are in .../lib, not .../libs.
* When using AC_CHECK_LIB, the -L option has to be put into LDFLAGS
  temporarily.  Use DUNE_CHECK_LIB_EXT, which does that automatically.
* Clear the output variables if GMP was not found.
* Put result into ALL_PKG_* instead of DUNE_PKG_*

[[Imported from SVN: r5852]]