Tentative refinement of ERT Autoconf support.

Add a new macro, "ERT", that uses AC_LINK_IFELSE to test if the ECL and
related libraries are available of the local computer system.  Call the
macro from "configure.ac".

Also, adapt the main "Makefile.am" to the results of the "ERT" macro.

m4/ert.m4

@@ -0,0 +1,67 @@
+AC_DEFUN([_ERT_SOURCE_TEXT],
+[
+AC_LANG_PROGRAM(
+[[
+#include <stddef.h>
+#include <ecl_util.h>
+]],dnl
+[[
+int sz;
+sz = ecl_util_get_sizeof_ctype(ECL_INT_TYPE);
+]])[]dnl
+])[]dnl
+
+# ----------------------------------------------------------------------
+
+AC_DEFUN([ERT],
+[
+AC_ARG_WITH([ert],
+            [AS_HELP_STRING([--with-ert=<root>], [Use ERT libraries])],
+            [], [with_ert=no])
+
+use_ert=no
+
+AS_IF([test x"${with_ert}" != x"no"],
+[
+  _ert_LDFLAGS_SAVE="${LDFLAGS}"
+  _ert_LIBS_SAVE="${LIBS}"
+  _ert_CPPFLAGS_SAVE="${CPPFLAGS}"
+  _ert_CFLAGS_SAVE="${CFLAGS}"
+
+  ERT_CPPFLAGS=
+  ERT_LDFLAGS=
+  ERT_LIBS="-lecl -lgeometry -lutil -lpthread -lz -lgomp"
+  AS_IF([test x"${with_ert}" != x"yes"],
+    [ERT_LDFLAGS="-L${with_ert}/lib"
+     ERT_CPPFLAGS="-I${with_ert}/include"], [:])[]dnl
+
+  CFLAGS="-std=gnu99"
+  CPPFLAGS="${ERT_CPPFLAGS} ${CPPFLAGS}"
+  LDFLAGS="${ERT_LDFLAGS} ${LDFLAGS}"
+  LIBS="${ERT_LIBS} ${LIBS}"
+
+  AC_LINK_IFELSE([_ERT_SOURCE_TEXT],
+    [use_ert=yes], [use_ert=no])
+
+  LIBS="${_ert_LIBS_SAVE}"
+  CPPFLAGS="${_ert_CPPFLAGS_SAVE}"
+  LDFLAGS="${_ert_LDFLAGS_SAVE}"
+  CFLAGS="${_ert_CFLAGS_SAVE}"
+
+  AS_IF([test x"${use_ert}" == x"yes"],
+   [AC_SUBST([ERT_CPPFLAGS])
+    AC_SUBST([ERT_LDFLAGS])
+    AC_SUBST([ERT_LIBS])
+    AC_DEFINE([HAVE_ERT], [1],
+              [Are the ERT libraries available for reading and writing ECLIPSE files.])],dnl
+   [:])[]dnl
+], [:])[]dnl
+
+AM_CONDITIONAL([HAVE_ERT], [test x"${use_ert}" != x"no"])
+
+# AC_MSG_ERROR(
+# [**** ERT_CPPFLAGS = ${ERT_CPPFLAGS} ****
+# **** ERT_LDFLAGS = ${ERT_LDFLAGS} ****
+# **** ERT_LIBS = ${ERT_LIBS} ****
+# ])
+])