download PCRE from sourceforge instead: ftp.pcre.org is dead
download PCRE from sourceforge instead: ftp.pcre.org is dead

Summary:
ftp.pcre.org is dead. We are currently migrating from PCRE1 to
PCRE2. In the meantime, download from SourceForge instead like we do
with Boost.

Reviewed By: akushner, xavierd

Differential Revision: D32520283

fbshipit-source-id: 9fda6930d7d8b677786a43419c4c0dc6289305f1

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