Revert to the canonical way of customizing CXXFLAGS

A default list of compiler flags is set via AM_CXXFLAGS Automake variable.
However this gets overridden by per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS in case the foo target). Because of this you should append
$(AM_CXXFLAGS) to the end of the per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS = ... $(AM_CXXFLAGS)).

After this default list of flags the contents of the CXXFLAGS user variable is
passed to the compiler. This variable has a default value determined by
configure (in case of g++ it is '-g -O2'). You can override this by specifying
CXXFLAGS when invoking make (e.g. make CXXFLAGS='-O3').

INCLUDE(FindPackageHandleStandardArgs)

FIND_PROGRAM(GHOSTSCRIPT_EXECUTABLE

  NAMES gs gswin32c

  PATHS "$ENV{ProgramFiles}/gs"

  PATH_SUFFIXES gs8.61/bin gs8.62/bin

  DOC "Ghostscript: PostScript and PDF language interpreter and previewer."

)

FIND_PACKAGE_HANDLE_STANDARD_ARGS(Ghostscript DEFAULT_MSG GHOSTSCRIPT_EXECUTABLE)