configure: allow 'strings' override via STRINGS variable

This allows building x264 on systems where 'strings' or
'${HOST}-strings' does not exist, but llvm-strings exists.
10 jobs for strings in 8 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #404758
aarch64 docker
build-debian-aarch64

00:07:43

passed #404757
amd64 docker
build-debian-amd64

00:01:26

passed #404761
macos
build-macos

00:01:30

passed #404759
amd64 docker
build-win32

00:02:03

passed #404760
amd64 docker
build-win64

00:02:20

 
  Test
passed #404763
aarch64 docker
test-debian-aarch64

00:00:24

passed #404762
amd64 docker
test-debian-amd64

00:00:15

passed #404766
macos
test-macos

00:00:05

passed #404764
amd64 docker
test-win32

00:00:17

passed #404765
amd64 docker
test-win64

00:00:16