Enable `pkg-config`-driven pycairo cflags

[jwilges]

Allow pkg-config to detect flags for either:

• py3cairo, or
• pycairo

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.

Resolves issue #234