Merge bitcoin/bitcoin#31357: cmake: Improve build script correctness

ab5c63e cmake: Build `secp256k1` only when required (Hennadii Stepanov)
76a3a54 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 ab5c63e
  tdb3:
    light test ACK ab5c63e

Tree-SHA512: 1b13f406c58b02768d9ba831413aeae1d7e03659e7101de8e598f906ba220f479ac06707965c96a14468ce4ba49011a1ab9adee9cee34ab1e8622f690b94dad8

src/CMakeLists.txt

@@ -73,6 +73,9 @@ if(DEFINED ENV{CFLAGS})
 endif()
 set(CMAKE_EXPORT_COMPILE_COMMANDS OFF)
 add_subdirectory(secp256k1)
+set_target_properties(secp256k1 PROPERTIES
+  EXCLUDE_FROM_ALL TRUE
+)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 string(APPEND CMAKE_C_COMPILE_OBJECT " ${APPEND_CPPFLAGS} ${APPEND_CFLAGS}")
 
@@ -296,8 +299,8 @@ target_link_libraries(bitcoin_node
     minisketch
     univalue
     Boost::headers
-    libevent::core
-    libevent::extra
+    $<TARGET_NAME_IF_EXISTS:libevent::core>
+    $<TARGET_NAME_IF_EXISTS:libevent::extra>
     $<TARGET_NAME_IF_EXISTS:libevent::pthreads>
     $<TARGET_NAME_IF_EXISTS:USDT::headers>
 )