OpenSystemsDevelopment / qpdf

This is needed to get an old enough version of glibc to run the Linux
binary as an AWS Lambda layer and to support some versions of CentOS.

Having it at compile time breaks cross-compilation and isn't really
right anyway.

m-holger's fix is better

This reverts commit fa3cdaf1b46dcbb583393f93a02f89207ba21a88.

If /AP is a dictionary of streams rather than a dictionary of
dictionaries, disregard /AS, which is supposed to point to a subkey of
one of the dictionaries. This fix prevents qpdf's annotation
flattening from discarding some annotations when /AS is erroneously
set.

Lazily load MD5 and RC4 once in the life of the program. Only load the
legacy provider if RC4 is actually being used.