The existing description did not properly account for ERT (or, more
appropriately, the "libecl" part of ERT) being installed in a
non-default location (e.g., somewhere below ${HOME}).
As EclipseGridParser.hpp declares various ERT-dependent types and
functions if a build configuration supports ERT (activated by the
`--with-ert' configure option), we need a proper include path to
reference the ERT headers. The fix is simple--just insert the
$(ERT_CPPFLAGS) into the already existing $(AM_CPPFLAGS).
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