Revert "build: exclude mptest target from compile commands" #35523

This reverts commit 4731049ba4f8a820cc4aa13a745e41bdfbee284a (#35418), which broke the build with -DBUILD_TESTS=OFF:

-- Performing Test HAVE_PTHREAD_GETTHREADID_NP - Failed
CMake Error at cmake/libmultiprocess.cmake:37 (set_target_properties):
  set_target_properties Can not find target to add properties to: mptest
Call Stack (most recent call first):
  src/CMakeLists.txt:24 (add_libmultiprocess)

Reported by afiore on IRC.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35523.

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tested ACK d186c390f4c7541dc08fe8573504d7082ff03415

I also ran into this error ~10hrs ago on Pop!_OS 24.04 LTS. With these changes, when I run a CMake configuration with:

cmake -B build -DBUILD_TESTS=OFF

the build files are generated without error

ACK d186c390f4c7541dc08fe8573504d7082ff03415, tested on Fedora 44.

If we follow the direction taken in #35418, the correct patch looks as follows:

--- a/cmake/libmultiprocess.cmake
+++ b/cmake/libmultiprocess.cmake
@@ -27,7 +27,10 @@ function(add_libmultiprocess subdir)
   mark_as_advanced(CapnProto_kj-tls_IMPORTED_LOCATION)
   if(BUILD_TESTS)
     # Add tests to "all" target so ctest can run them
-    set_target_properties(mptest PROPERTIES EXCLUDE_FROM_ALL OFF)
+    set_target_properties(mptest PROPERTIES
+      EXCLUDE_FROM_ALL OFF
+      EXPORT_COMPILE_COMMANDS OFF
+    )
   endif()
   # Exclude examples from compilation database, because the examples are not
   # built by default, and they contain generated c++ code. Without this

However, I still prefer #35418#pullrequestreview-4428862711.