I would expect make to remove trailing comments and then strip surrounding white space from variable definitions as usual but it seems that the comments prevent further stripping:
$ cat Makefile
# a classic var
FOO = foo
BAR = bar # and another but commented differently
echo_some_vars:
@echo "X$(FOO)X"
@echo "X$(BAR)X"
---------------------------------------------------------------------------
$ make echo_some_vars
XfooX
Xbar X
