The previous method of always using sys.exec_prefix to locate pycairo
headers is insufficient on systems where Python and pycairo have
different install prefixes (e.g. /usr vs. /usr/local).
For backward compatibility, the previous method is still the default
behavior when pkg-config cannot detect appropriate flags at runtime.
Allow pkg-config to detect flags for either:
py3cairo, orpycairoThe previous method of always using
sys.exec_prefixto locate pycairo headers is insufficient on systems where Python and pycairo have different install prefixes (e.g./usrvs./usr/local).For backward compatibility, the previous method is still the default behavior when
pkg-configcannot detect appropriate flags at runtime.Resolves issue #234