fix(makefile): Fix typo and use POSIX comparison in the makefile (#443)
# What does this PR do?
This PR fixes:
- The usage of non posix comparison which may fail depending on the
shell used (`=` will always work, `==` only with bash)
- Typo in the env variable name displayed in the error message
`BUILD_EXTENSION` instead of `BUILD_EXTENSIONS`
<!-- Remove if not applicable -->
Fixes #422