Makefile: fix stray \ warnings with grep-3.8
We simply grep for "/usr". So no need for "-E" or "\/". Furthermore, in
the new grep versions this creates warnings.
As written in the grep-3.8 announcement:
Regular expressions with stray backslashes now cause warnings, as
their unspecified behavior can lead to unexpected results.
For example, '\a' and 'a' are not always equivalent
<https://bugs.gnu.org/39678>.
Fixes warnings in the form of:
grep: warning: stray \ before /
Signed-off-by: Nick Hainke <[email protected]>
Nick Hainke