Update prepare.sh to resolve gcc error during CRAS compile

[mkststrahilov]

Fix for #4923

CRAS fails to compile due to gcc error, a temporary fix was proposed via targets/audio to omit the wrong commands but could not get it to work really... instead implemented the fix directly to convert_automake and it complied without issue.

First time doing a pull request, so if doing something wrong - please have mercy

[drinkcat]

Instead of this, can you edit Makefile.am directly?

The idea is to remove this line (https://chromium.googlesource.com/chromiumos/third_party/adhd/+/c2ece09f69565afcbcf511b1693f3fe735bde511/cras/src/Makefile.am#50):

COMMON_CPPFLAGS += -Werror=undef-prefix -Wundef-prefix=HAVE_,CRAS_

So something like this will do, around here (https://github.com/dnschneid/crouton/blob/master/targets/audio#L165):

	sed -i -e 's/^.*error=undef-prefix.*$//'  Makefile.am

(it's a bit less strict that necessary but that's ok for now, and we have bigger problems incoming..., see #4958)

[drinkcat]

Or as @dnschneid mentioned:

	sed -i -e '/error=undef-prefix/d'  Makefile.am

[mkststrahilov]

Instead of this, can you edit Makefile.am directly?

The idea is to remove this line (https://chromium.googlesource.com/chromiumos/third_party/adhd/+/c2ece09f69565afcbcf511b1693f3fe735bde511/cras/src/Makefile.am#50):

COMMON_CPPFLAGS += -Werror=undef-prefix -Wundef-prefix=HAVE_,CRAS_

So something like this will do, around here (https://github.com/dnschneid/crouton/blob/master/targets/audio#L165):

	sed -i -e 's/^.*error=undef-prefix.*$//'  Makefile.am

(it's a bit less strict that necessary but that's ok for now, and we have bigger problems incoming..., see #4958)

I can try to play around with it tomorrow and see if it works fine. Just learned about sed yesterday, been reading documentation left-right to figure what to do, cause I wanted to try Crouton out of curiosity. But hey, learning something new ain't bad hehe.

Just checked #4958... oh boy.

Note to myself - what is Bazel... why is bazel