configure: Provide a safe default for unknonw libcs

Support musl out of box.

Amend commit 9d2cee52d37c7340f85a5d41110282aac03e6855 that got
wrongly merged in its stead.
Merge commit '551746680462538bb5ad7adf55fb1248190428ec'

* commit '551746680462538bb5ad7adf55fb1248190428ec':
  configure: Provide a safe default for unknonw libcs

Conflicts:
	configure

This is not merged as it does not work with musl here, while
our previous solution does.

See: 170e2fd7c39c430b334ea80a1e4dfb10c6fd1e95
Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/ac3enc_template: fix out of array read

Found-by: Andreas Cadhalpun
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>