ab5c63edcc cmake: Build `secp256k1` only when required (Hennadii Stepanov)
76a3a540a4 cmake: Ensure script correctness when no targets are specified (Hennadii Stepanov)
Pull request description:
When no build targets are specified, it is reasonable to expect the configuration step to succeed and produce a build system that does not build any targets.
This PR updates the code to ensure this behaviour:
```
$ cmake -B build -G "Ninja" -DBUILD_DAEMON=OFF -DBUILD_CLI=OFF -DBUILD_TX=OFF -DBUILD_UTIL=OFF -DENABLE_WALLET=OFF -DBUILD_TESTS=OFF
$ cmake --build build
ninja: no work to do.
```
ACKs for top commit:
TheCharlatan:
ACK ab5c63edcc
tdb3:
light test ACK ab5c63edcc
Tree-SHA512: 1b13f406c58b02768d9ba831413aeae1d7e03659e7101de8e598f906ba220f479ac06707965c96a14468ce4ba49011a1ab9adee9cee34ab1e8622f690b94dad8
abeebccc48