External libraries for Windows are now built automatically in the
qpdf/external-libs repository and include openssl in addition to zlib
and jpeg. Use these, and update the Windows build to build with the
openssl crypto provider by default. We leave the native crypto
provider enabled in case there is a problem with openssl and also to
continue to exercise that code.
Replace the old msys, mingw-w64, ActiveState perl setup with MSYS2.
This greatly simplifies the Windows build. This was facilitated by the
need to incorporate JPEG.
Make --enable-werror work properly on msvc, handle extra warnings
flags for msvc in configure.ac instead of hardcoding into
make/msvc.mk, separate warnings flags into WFLAGS in autoconf.mk to
avoid duplication and to make it easier to override.
Update the build to support new external-libs layout, and autoconf
options to specify windows word size. Split make_windows_releases
into multiple scripts.
