Rewrite the DUNE_CHECK_LIB_EXT macro:

 * Essentially copy and adapt the definition of AC_CHECK_LIB.  This should
   make sure the two macros behave the same even in corner cases (except for
   the added functionality, of course).

 * Properly document what exactly is allowed in the arguments.

Parse arguments to DUNE_CHECK_LIBS_EXT properly in parmetis.  This avoids
expansions like 'LDFLAGS="$LDFLAGS -L"$PARMETISLIBPATH""'.

[[Imported from SVN: r5830]]

m4/dune.m4

@@ -194,10 +194,10 @@ AC_DEFUN([DUNE_CHECK_MODULES],[
       _DUNE_MODULE[]_CPPFLAGS="`$PKG_CONFIG --cflags _dune_name`" 2>/dev/null
       _DUNE_MODULE[]_ROOT="`$PKG_CONFIG --variable=prefix _dune_name`" 2>/dev/null 
       _DUNE_MODULE[]_VERSION="`$PKG_CONFIG --modversion _dune_name`" 2>/dev/null
-      ifelse(_dune_symbol,,,[
-        _DUNE_MODULE[]_LDFLAGS="-L`$PKG_CONFIG --variable=libdir _dune_name`" 2>/dev/null 
-        _DUNE_MODULE[]_LIBS="-l[]_dune_lib"
-      ])
+      _DUNE_MODULE[]_LDFLAGS=""
+      ifelse(_dune_symbol,,
+        [_DUNE_MODULE[]_LIBS=""],
+        [_DUNE_MODULE[]_LIBS="-L`$PKG_CONFIG --variable=libdir _dune_name 2>/dev/null` -l[]_dune_lib"])
       HAVE_[]_DUNE_MODULE=1
       AC_MSG_RESULT([global installation in $_DUNE_MODULE[]_ROOT])
     ],[
@@ -227,10 +227,10 @@ AC_DEFUN([DUNE_CHECK_MODULES],[
         _DUNE_MODULE[]_CPPFLAGS="-I$_DUNE_MODULE[]_SRCDIR"
         _DUNE_MODULE[]_VERSION="`grep Version $_DUNE_MODULE[]_SRCDIR/dune.module | sed -e 's/^Version: *//'`" 2>/dev/null
       ])
-      ifelse(_dune_symbol,,,[
-        _DUNE_MODULE[]_LDFLAGS="-L$_DUNE_MODULE[]_ROOT/lib"
-        _DUNE_MODULE[]_LIBS="-l[]_dune_lib"
-      ])
+      _DUNE_MODULE[]_LDFLAGS=""
+      ifelse(_dune_symbol,,
+        [_DUNE_MODULE[]_LIBS=""],
+        [_DUNE_MODULE[]_LIBS="-L$_DUNE_MODULE[]_ROOT/lib -l[]_dune_lib"])
       # set expanded module path
       with_[]_dune_module="$_DUNE_MODULE[]_ROOT"
       HAVE_[]_DUNE_MODULE=1

m4/parmetis.m4

@@ -85,7 +85,7 @@ AC_DEFUN([DUNE_PATH_PARMETIS],[
       LIBS="$DUNEMPILIBS -lm $LIBS"
       
       if test x$HAVE_PARMETIS = x1 ; then
-	  DUNE_CHECK_LIB_EXT(["$PARMETIS_LIB_PATH"], [metis], [metis_partgraphkway],
+	  DUNE_CHECK_LIB_EXT([$PARMETIS_LIB_PATH], [metis], [metis_partgraphkway],
               [
 		  PARMETIS_LIBS="-L$PARMETIS_LIB_PATH -lmetis $DUNEMPILIBS -lm"
 		  LIBS="$PARMETIS_LIBS $ac_save_LIBS"
@@ -96,7 +96,7 @@ AC_DEFUN([DUNE_PATH_PARMETIS],[
       fi
 
       if test x$HAVE_PARMETIS = x1 ; then
-	  DUNE_CHECK_LIB_EXT(["$PARMETIS_LIB_PATH"], [parmetis], [parmetis_v3_partkway],
+	  DUNE_CHECK_LIB_EXT([$PARMETIS_LIB_PATH], [parmetis], [parmetis_v3_partkway],
               [
 		  PARMETIS_LIBS="-L$PARMETIS_LIB_PATH -lparmetis -lmetis $DUNEMPILIBS -lm"
               ],[