configure.ac: Add --with-crypto= instead of many different --with-$backend
The new --with-crypto option replaces the previous backend-specific
--with-{openssl,libgcrypt,mbedtls,wincng} options and fixes some issues.
* libgcrypt or mbedtls would previously be used whenever found, even
if configure was passed --without-libgcrypt or --without-mbedtls.
* If --with-$backend was specified then configure would not fail even
if that library could not be found, and would instead use whichever
crypto library was found first.
The new option defaults to `auto`, which makes configure check for all
supported crypto libraries in turn, choosing the first one found, or
exiting with an error if none can be found.
8 files changed
