Build system --enable options

[Hendi]

Hi,

I want to build a minimal libgcrypt for a resource-starved embedded system.

The configure script offers various options such as --enable-digest, 
--enable-kdfs, etc., however there are two problems:
* It doesn't seem possible to enable nothing, as an empty string is 
interpreted as "everything". In particular, if I 
specify --enable-kdfs="", I get all KDF algorithms.
* It's impossible to build libgcrypt with only sha1 and sha256, as this 
causes a linking failure. With the current code base, sha512, sha3 and 
blake2 are mandatory.

It would also be great if the KEM stuff that was added recently could be 
disabled with a compile-time flag, as the Poly1305/McEliece/Kyber 
algorithms take dozens of kilobytes in the library.

Best regards,
Hendi