Bugfix for beaa1a44554d04d9fe63a743a5bb4431ca778f28 (#9235)

Bugfix for beaa1a44554d04d9fe63a743a5bb4431ca778f28 (#9235)

[build:Makefile] Restore compatibility with GNU Make <4.0

- The != variable assignment operator is not supported by GNU Make <4.0
- $(shell) is a no-op in BSD Make, assigns an empty string to the var
- Try to assign with != and fallback to $(shell) if not assigned (?=)

- Old versions of BSD find have different -exec behavior
- Pipe to `sed` instead of using `find ... -exec dirname {}`

- BSD tar does not support --transform, --owner or --group
- Allow user to specify path to GNU tar by passing GNUTAR variable

- pandoc vars are immediately evaluated with != in gmake>=4 and bmake
- Suppress stderr output for pandoc -v in case it is not installed
- Use string comparison instead of int comparison for pandoc version

Authored by: bashonly