Improve AX_AT_CHECK_PATTERN() quoting

Use m4_expand([$1]) instead of bare $1 when calling AT_CHECK()
from AX_AT_CHECK_PATTERN().
Fixes problems observed when pattern has a comma in it.
Add test suite for the issue (which failed before I fixed the .m4).
Extend AX_AT_DATA_CHECK_PATTERN_AWK() header

Add more comments to the header in the file generated
by AX_AT_DATA_CHECK_PATTERN_AWK() describing what the
awk script does.
Release ax_at_check_pattern 4

* Provide AX_AT_DATA_CHECK_PATTERN_AWK() to write an awk script
  containing the diff post-processor to a file.
* Add README.
* Improve quoting and use quadrigraphs.
* Add references to github.
* Extend testsuite for AX_AT_DATA_CHECK_PATTERN_AWK() and quoting fixes.