Escape number sign in Makefiles

Number signs are handled differently in Makefile variable parsing as
compared to bash variable parsing. See this demo:

```
$ cat Makefile
A=foo#bar
B='foo#bar'
C="foo#bar"
D=foo\#bar
E='foo\#bar'
F="foo\#bar"

$(info $(A))
$(info $(B))
$(info $(C))
$(info $(D))
$(info $(E))
$(info $(F))

$ make
foo
'foo
"foo
foo#bar
'foo#bar'
"foo#bar"
make: *** No targets.  Stop.

$ make -v
GNU Make 4.2.1
```

In other words, the `#` character is evaluated first when parsing
Makefiles, causing the rest of the line to become a comment. The effect of
this is that paths that contain embedded `#` symbols are not handled
properly in the vpx build system.

To test this change, clone vpx to a directory containing a `#` symbol and
attempt a build. With this change, it worked for me on Fedora 31, however
without the change the build failed.

Change-Id: Iaee6383e2435049b680484cc5cefdea9f2d9df46

0 files changed, 0 insertions, 0 deletions