AVRO-4107: [C++] Remove boost::algorithm

[wgtmac]

What is the purpose of the change

Remove boost::algorithm

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? no

[KalleOlaviNiemitalo]

If buf[start] or buf[end - 1] is a negative char other than EOF, then passing it to std::isspace(int ch) causes undefined behavior. To avoid the risk, this should cast the char values to unsigned char.

(In principle, this might also be expected to recognize and trim multibyte whitespace characters as defined by the current locale. But it seems unlikely that such characters would actually be used in the preprocessor directives for which this function searches. And boost::algorithm::trim doesn't seem to recognize them either, as it checks each char individually.)

[wgtmac]

I'm not sure if it is better to call std::isspace(buf[start], std::locale::classic()) instead of std::isspace(static_cast<unsigned char>(buf[start]))

[wgtmac]

@thiru-mg @martin-g @Fokko Could you help review this?

[thiru-mg]

@wgtmac @KalleOlaviNiemitalo

I think the version that @wgtmac implemented (which uses locale) is the closest to what boost implemented. Boost's trim also uses the current locale for determining if something is a space.