This PR replaces the bundled libgd includes from #include <foo.h> with
#include "foo.h" for gd-related headers to avoid including headers that
may be available in system directories instead of the expected local
headers.
Closes GH-6975.
Even though libgd/libgd#492 is not a relevant bug fix for PHP, since
the binding doesn't use the `gdImage*Ptr()` functions at all, we're
porting the fix to stay in sync here.
There's no need anymore to call an own error handler directly. Instead we
register our error handler and call libgd's error functions (which will
forward). We do this regardless of compiling with the bundled or an external
libgd.
This initial checkin has no changes to any of the libgd code so it can
be used as a basis for diffs. It also will not build currently because
of this. The PHP gd checks need to be incorporated along with a bit of
other config magic. It also shouldn't break the build and will only
take effect if you use --with-gd=php right now.
