Unify the many copies of diff_with_file in one source
file, and patch it to detect diff at runtime and fall
back to a simple strcmp if we don't have it. Make all
tests use this new testutils.c, and stop requiring
diff for building the tests.
This should let us allow to build on Windows with the
default value for -Dbuild-tests.
Properly handle diff(1) failing.
In this particular case, the test passed a NULL input file to the diff
(that was fixed, too) and then diff only found one input file and
aborted.
But without this fix, we'd also not catch other abortion reasons for
diff() - as long as it exited in any way, we were happy.
Instead of just checking that the line of the error message is correct,
assert that start and end position are on the correct character offset.
Also fix all the tests to conform to this.
This library is meant to be the new CSS library that gets used from GDK,
GSK and GTK for string printing and parsing.
As a first step, move GtkCssProviderError into it.
While doing so, split it into GtkCssParserError (for critical problems)
and GtkCssParserWarning (for non-critical problems).
