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 

Makefile

@@ -2,7 +2,7 @@ install-server:
 	cd server && make install
 
 install-custom-kernels:
-	if [ "$$BUILD_EXTENSIONS" == "True" ]; then cd server/custom_kernels && python setup.py install; else echo "Custom kernels are disabled, you need set to BUILD_EXTENSION environment variable to 'True' in order to build them. (Please read the docs, kernels might not work on all hardware)"; fi
+	if [ "$$BUILD_EXTENSIONS" = "True" ]; then cd server/custom_kernels && python setup.py install; else echo "Custom kernels are disabled, you need to set the BUILD_EXTENSIONS environment variable to 'True' in order to build them. (Please read the docs, kernels might not work on all hardware)"; fi
 
 install-integration-tests:
 	cd integration-tests && pip install -r requirements.txt