Given the unreliability of using system libraries, nginx really does do it right by building its own PCRE, OpenSSL, and zlib. However, lots of extraneous stuff gets built for these items (such as manpages). This drastically increases nginx build time, particularly for the OpenSSL part.
Just try building nginx repeatedly for debugging purposes, and watching those openssl manpage installations scroll by again and again, eating precious iops and wall-clock time for no reason whatsoever---complete waste!
If the nginx make system ran configure on its libraries to omit the all stuff nginx doesn't actually need, this would not waste resources, and would significantly speed up build times. Really, nginx should tell configure to just build the static libraries.
Mr. Sysoev, in the long term, the cumulative time you would save yourself on test builds would probably more than make up for the time you spend fixing this. ;-)
Thanks.