Configure, qnx hints: Improve the cross-compilation support for QNX

Now, like with Android and plain Linux, cross-compiling to QNX (which
includes newer Blackberry devices) will load hints/qnx.sh.
t/io/fs.t: sh in Blackberry might behave like android's
Configure, Solaris hints: Allow cross-compiling to Solaris.

This only required some minor changes to the Solaris hints, and
adding a few lines for Solaris in the part of Configure that handles
setting osname and osvers when cross-compiling.
README.solaris: Add a section for cross-compilation
Configure: Added a targetenv variable

Currently this is only used during make test, and allows setting
the environment of the target before running tests.
Glossary for targetenv
QNX hints: Add LC_ALL=C to targetenv
README.qnx: Add a section for cross-compilation
Configure: use $undef, not plain undef
INSTALL: Updates to the cross-compilation section