Complete rely on pkg-config to figure out the correct flags for ipopt.

Previously parts of the flags were hand-coded in ipopt.m4.
That lead to difficult-to-interpret compiler, linker, and
run-time bugs.  Taking everything from pkg-config makes these
issues go away.  Touch wood.

One consequene is that the include paths into IPOpt need to
change.  But that shouldn't really regard anyone but me.

[[Imported from SVN: r7295]]