Sed

echo "GLFW_INCL_DIR := bar" | sed 's/bar$//'

sed --expression="s:#SSL_TREE =.*:SSL_TREE=$(OPENSSL_DIR):"

 GLFW_INCL_DIR := 

echo "GLFW_INCL_DIR := bar" | sed 's/\(GLFW_INCL_DIR :=\).*/\1/'

echo "GLFW_INCL_DIR := bar" | sed 's/^\(GLFW_INCL_DIR :=\).*$/\1/'