Poppler compiler option change

[mpope9]

The building wiki is a bit out of date it seems.

The new compiler options for poppler's xpdf are -DENABLE_UNSTABLE_API_ABI_HEADERS=ON, instead of --enable-xpdf-headers.

Not sure which version of poppler made the change, and it isn't in the INSTALL file provided by them. But is found by grepping their cmake options:

> egrep '^ *(option|set.*STRING)' CMakeLists.txt
option(ENABLE_UNSTABLE_API_ABI_HEADERS "Install API/ABI unstable xpdf headers." OFF)
...

[mpope9]

PR https://github.com/pdf2htmlEX/pdf2htmlEX/pull/39 wraps everything up. I didn't see that docker was recommended first. Maybe it should be directly in the README that docker is the easiest solution.