fix GMP test (FS#811)

[[Imported from SVN: r6102]]

m4/gmp.m4

@@ -24,35 +24,21 @@ AC_DEFUN([DUNE_PATH_GMP],[
 
   AC_ARG_WITH([gmp],
     [AS_HELP_STRING([--with-gmp=PATH],
-                   [directory to GMP (GNU MP Bignum Library)])],
-    [],
-    [with_gmp=/usr])
+                   [directory to GMP (GNU MP Bignum Library)])])
 
   HAVE_GMP=no
   AS_IF([test x"$with_gmp" = x"no"], [
     AC_MSG_NOTICE([skipping check for GMP])
   ], [
-    GMP_CPPFLAGS="-I$with_gmp/include -DENABLE_GMP=1"
-    GMP_LIBS="-L$with_gmp/lib -lgmpxx -lgmp"
-
-    AC_LANG_PUSH([C++])
-    ac_save_CPPFLAGS="$CPPFLAGS"
-    ac_save_LIBS="$LIBS"
-
-    CPPFLAGS="$CPPFLAGS $GMP_CPPFLAGS"
-
-    AC_CHECK_HEADER([gmpxx.h], [HAVE_GMP=yes],
-      [AC_MSG_WARN([gmpxx.h not found in $with_gmp])])
-
-    AS_IF([test $HAVE_GMP = yes],[
-      DUNE_CHECK_LIB_EXT([$with_gmp/lib], gmp,[__gmpz_abs],[],[
-        HAVE_GMP=no
-        AC_MSG_WARN(libgmp not found)])
+    AS_IF([test x"$with_gmp" = x || test x"$with_gmp" = xyes], [
+      for d in /usr /usr/local; do
+        AC_MSG_NOTICE([searching for GMP in $d...])
+        DUNE_CHECK_PATH_GMP($d)
+        AS_IF([test $HAVE_GMP = yes],[break])
+      done],[
+      DUNE_CHECK_PATH_GMP($with_gmp)
     ])
 
-    CPPFLAGS="$ac_save_CPPFLAGS"
-    LIBS="$ac_save_LIBS"
-    AC_LANG_POP([C++])
   ])
 
   AS_IF([test $HAVE_GMP = yes],[
@@ -70,3 +56,28 @@ AC_DEFUN([DUNE_PATH_GMP],[
   AM_CONDITIONAL(GMP,[test $HAVE_GMP = yes])
   DUNE_ADD_SUMMARY_ENTRY([GMP],[$HAVE_GMP])
 ])
+
+AC_DEFUN([DUNE_CHECK_PATH_GMP],[
+  GMP_CPPFLAGS="-I$1/include -DENABLE_GMP=1"
+  GMP_LIBS="-L$1/lib -lgmpxx -lgmp"
+
+  AC_LANG_PUSH([C++])
+  ac_save_CPPFLAGS="$CPPFLAGS"
+  ac_save_LIBS="$LIBS"
+
+  CPPFLAGS="$CPPFLAGS $GMP_CPPFLAGS"
+
+  AC_CHECK_HEADER([gmpxx.h], [HAVE_GMP=yes],
+    [AC_MSG_WARN([gmpxx.h not found in $1])])
+
+  AS_IF([test $HAVE_GMP = yes],[
+    DUNE_CHECK_LIB_EXT([$1/lib], gmp,[__gmpz_abs],[],[
+      HAVE_GMP=no
+      AC_MSG_WARN(libgmp not found)])
+  ])
+
+  CPPFLAGS="$ac_save_CPPFLAGS"
+  LIBS="$ac_save_LIBS"
+  AC_LANG_POP([C++])
+])
+