Many of Pango's dependencies do not support a build system for Visual
Studio that would generate the pkg-config files for them, so we need to
try to look for them using cc.has_header() and cc.find_library() for
them, namely for Cairo, FreeType, FontConfig and HarfBuzz as fallbacks.

For Cairo, things are more complicated as there are multiple build
options and configurations for it, so we need to check for those that we
need after we find the Cairo headers and libraries by:

-Including the respective headers (for cairo-win32.h, cairo-ps.h,
 cairo-pdf.h, cairo-quartz.h and cairo-xlib.h, since these features
 must have been enabled when Cairo is built and installed in order for
 those headers to be succesfully included)
-For pangocairo with FreeType support, we need to check whether the
 FontConfig support is built into Cairo as well, as FontConfig support
 is actually required in Cairo for this.
-For Cairo/PNG output surface support, check whether Cairo is built with
 PNG output surface support.

We also need to update how pangocairo.pc and pangoft2 are generated:
-On builds where pkg-config files can be found for cairo, freetype,
 fontconfig and/or harfbuzz:
 Generate it with "Requires: pango <depedencies>" as before, otherwise
 for MSVC builds where we find these libraries manually, we don't put
 these packages under "Requires:..." or "Requires.private:...", but
 instead put them under "Libs:", linking to each dep as -l<.lib file
 name>

This is so that pangocairo.pc and pangooft2.pc can be correctly used by
items that need to make use of it, such as g-ir-scanner.

https://bugzilla.gnome.org/show_bug.cgi?id=783274