Merge branch 'develop'

Merge branch 'develop'

Merge pull request boostorg#245 from boostorg/develop

Suppressing -Wsign-conversion warnings

Merge pull request boostorg#245 from boostorg/develop

Suppressing -Wsign-conversion warnings

Revert "Select assembly sources via preprocessor"

This reverts commit 2fb0373.

Select assembly sources via preprocessor

Fixes cross-compilation abi and binformat detection.
`abi` and `binary-format` are now optional but can be used
to override autodetection.

Merge branch 'develop'

Merge branch 'develop'

Merge branch 'develop'

Merge branch 'develop'