irssi: Fix OpenSSL without deprecated APIs compilation

irssi: Fix OpenSSL without deprecated APIs compilation

When OpenSSL is built without deprecated APIs, pkg-config first tries
OpenSSL in the staging directory but fails as it cannot find the
deprecated SSL_library_init function and ends up finding the system one.

Added PKG_BUILD_PARALLEL for faster compilation.

Added -Wl,--gc-sections to LDFLAGS to save ~10KB from the resulting ipk.

Reworked configure section as some of those options were renamed or
removed.

Removed EXTRA_* hacks that are no longer necessary.

Signed-off-by: Rosen Penev <rosenp@gmail.com>