Don't try to link against ipopt if pkg-config already found it.

Because we trust pkg-config.  If it says that ipopt is there,
then it is there.

[[Imported from SVN: r7939]]

m4/ipopt.m4

@@ -47,19 +47,6 @@ AS_IF([test x$with_ipopt != xno], [
     CPPFLAGS="$CPPFLAGS $IPOPT_CPPFLAGS"
     LIBS="$LIBS $IPOPT_LIBS"
 
-    AC_LANG_PUSH([C++])
-
-    # if header is found...
-    AS_IF([test x$HAVE_IPOPT = x1], [
-       AC_LINK_IFELSE([AC_LANG_PROGRAM(
-	    [#include "IpIpoptApplication.hpp"],
-	    [Ipopt::SmartPtr<Ipopt::IpoptApplication> app = new Ipopt::IpoptApplication();])],
-	[],
-	[HAVE_IPOPT="0"
-	AC_MSG_WARN(IPOpt does not seem to work!)])
-    ])
-
-    AC_LANG_POP([C++])
 ])
 
 with_ipopt="no"