Check for cross compiling. If cross compiling, build stage1flex using a
custom link command.
We also override LDADD since that adds the replacement implementations
that are cross compiled, and instead always use the replacement library
implementations.

We don't use BUILD_OBJEXT and BUILD_EXEEXT since it seems that automake
does not support these.

Fixes #78.

The .cvsignore files from the legacy cvs repository tracked what files
got autogenerated during various stages of the flex build. Renaming the
.cvsignore files to .gitignore lets git do the same thing. git is better
about letting higher level .gitignore files not-track files in lower
level directories. As I work my way through the test directories, we may
add additional .gitignore files from the old .cvsignore files.

The po/ directory has a lot of special files used by gettext, so the patterns in
po/.gitignore look very different.

The doc/.gitignore file accounts for
what texinfo/makeinfo do, and so it also has special patterns.

The m4 directory is mainly present for autoconf's benefit, but we have
to account for it so make can do the right thing. Hence, m4/.gitignore
says to ignore *.m4, as counterintuitive as that may seem.