Further incompatibilities with BSD sed

1. BSD sed does not support the escaped alternation symbol, i.e. \|, but only the unescaped version, i.e. |, with the -E flag (some time in between MacOS 10.15.5 and MacOS 12.3.1, the alternative -r flag got the support as well). In particular, this affects:

   • tests/test_exported_symbols.in;
   • m4/acx_use_libtool_configuration.m4 (I have to admit that 837c625d was the right way to solve the problem and 78563389, which was adopted as d820b818, was not).

2. BSD sed does not support the word boundary sequence \b. In particular, this affects:

   • the same m4/acx_use_libtool_configuration.m4;
   • and contrib/06ltmain_nag_pthread-patch/ltmain_nag_pthread.patch (one of the things that breaks building with NAG on MacOS).

   It looks like we could replace \b with the end of a word sequence in the two cases above. The only problem is that the end of the word sequence is /> for GNU sed and [[:>:]] for BSD sed.

I have not checked whether there are more lines in the code affected by these problems.