Merge bitcoin/bitcoin#30814: kernel: Create usable static kernel library

0dd16d7 build: Add a pkg-config file for libbitcoinkernel (TheCharlatan)
45be32f build: Produce a usable static kernel library (TheCharlatan)

Pull request description:

  Since the move to cmake, the kernel static library that is installed after a cmake --install build is unusable. It lacks symbols for the internal libraries, besides those defined in the kernel library target.

  Fix this by explicitly installing all the required internal static libraries. To make usage of these installed libraries easy, add a pkg-config file that can be used during linking.

  This patch can be tested with:

  ```
  cmake -B build -DBUILD_SHARED_LIBS=OFF -DBUILD_KERNEL_LIB=ON
  cmake --build build
  cmake --install build
  g++ -std=c++20 -o test_chainstate src/bitcoin-chainstate.cpp -I/home/drgrid/bitcoin/src $(pkg-config --libs --static libbitcoinkernel)
  ```

  Attempts to solve #30801

ACKs for top commit:
  hebasto:
    ACK 0dd16d7.
  fanquake:
    ACK 0dd16d7 - this looks like a good place to start.
  ryanofsky:
    Code review ACK 0dd16d7

Tree-SHA512: 92f7bc959584bdc595f4aa6d0ab133355481075fe8564224fd7ac122fd7bdd75f98cf26ef0a6a7d84fd552d2258ddca1b674eca91122469a58bacc5f0a0ec2ef

Author: fanquake

libbitcoinkernel.pc.in

@@ -0,0 +1,11 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+
+Name: @PACKAGE_NAME@ kernel library
+Description: Experimental library for the Bitcoin Core validation engine.
+Version: @PACKAGE_VERSION@
+Libs: -L${libdir} -lbitcoinkernel
+Libs.private: -L${libdir} @LIBS_PRIVATE@
+Cflags: -I${includedir}

src/kernel/CMakeLists.txt

@@ -98,6 +98,40 @@ set_target_properties(bitcoinkernel PROPERTIES
   CXX_VISIBILITY_PRESET default
 )
 
+# When building the static library, install all static libraries the
+# bitcoinkernel depends on.
+if(NOT BUILD_SHARED_LIBS)
+  # Recursively get all the static libraries a target depends on and put them in libs_out
+  function(get_target_static_link_libs target libs_out)
+    get_target_property(linked_libraries ${target} LINK_LIBRARIES)
+    foreach(dep ${linked_libraries})
+      if(TARGET ${dep})
+        get_target_property(dep_type ${dep} TYPE)
+        if(dep_type STREQUAL "STATIC_LIBRARY")
+          list(APPEND ${libs_out} ${dep})
+          get_target_static_link_libs(${dep} ${libs_out})
+        endif()
+      endif()
+    endforeach()
+    set(${libs_out} ${${libs_out}} PARENT_SCOPE)
+  endfunction()
+
+  set(all_kernel_static_link_libs "")
+  get_target_static_link_libs(bitcoinkernel all_kernel_static_link_libs)
+
+  # LIBS_PRIVATE is substituted in the pkg-config file.
+  set(LIBS_PRIVATE "")
+  foreach(lib ${all_kernel_static_link_libs})
+    install(TARGETS ${lib} ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
+    string(APPEND LIBS_PRIVATE " -l${lib}")
+  endforeach()
+
+  string(STRIP "${LIBS_PRIVATE}" LIBS_PRIVATE)
+endif()
+
+configure_file(${PROJECT_SOURCE_DIR}/libbitcoinkernel.pc.in ${PROJECT_BINARY_DIR}/libbitcoinkernel.pc @ONLY)
+install(FILES ${PROJECT_BINARY_DIR}/libbitcoinkernel.pc DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+
 include(GNUInstallDirs)
 install(TARGETS bitcoinkernel
   RUNTIME