[change|remove]_make_var: mind leading blanks.

Not all Makefiles are well formed...

Some prepend blanks before defining variables and `change_make_var`/`remove_make_var` currently don't detect that. This is the cause of a current bug with `suite-sparse` in Homebrew/Science where the variable `BLAS` is defined in two places:
````
$ egrep '^[ ]*BLAS' SuiteSparse_config_Mac.mk
BLAS = -lblas -lgfortran
  BLAS = -framework Accelerate
````
The result is that if users build with `--with-openblas`, only the first `BLAS` gets replaced, but it's still Accelerate that's being used!