HACKING.md: explain C platform macros from https://cfengine.com/dev/issues/2018
.gitignore: in addition to ignoring "TAGS" ignore "tags".
Merge pull request #1025 from cdituri/cdituri/core/gitignore-tags

.gitignore: in addition to ignoring "TAGS" ignore "tags".
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Merge pull request #1027 from basvandervlies/spelling_error_avaiable

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Merge pull request #1022 from tzz/feature/2018-explain-platform-macros

HACKING.md: explain C platform macros from https://cfengine.com/dev/issues/2018