shell - How to source a script in a Makefile?

Question

Is there a better way to source a script, which sets env vars, from within a makefile?

FLAG ?= 0
ifeq ($(FLAG),0)
export FLAG=1
/bin/myshell -c '<source scripts here> ; $(MAKE) $@'
else
...targets...
endif

Answer 1

Makefile default shell is /bin/sh which does not implement source.

Changing shell to /bin/bash makes it possible:

# Makefile

SHELL := /bin/bash

rule:
    source env.sh && YourCommand