Move CMake directory (#4997)

cmake/target_protobuf_sources.cmake

@@ -0,0 +1,62 @@
+find_package(Protobuf REQUIRED)
+
+# .proto files import each other like this:
+#
+# import "path/to/file.proto";
+#
+# For the protobuf compiler to find these imports,
+# the parent directory of "path" must be in the import path.
+#
+# When generating C++,
+# it turns into an include statement like this:
+#
+# #include "path/to/file.pb.h"
+#
+# and the header is generated at a path relative to the output directory
+# that matches the given .proto path relative to the source directory
+# minus the first matching prefix on the import path.
+#
+# In other words, a file `include/package/path/to/file.proto`
+# with import path [`include/package`, `include`]
+# will generate files `output/path/to/file.pb.{h,cc}`
+# with includes like `#include "path/to/file.pb.h".
+#
+# During build, the generated files can find each other if the output
+# directory is an include directory, but we want to install that directory
+# under our package's include directory (`include/package`), not as a sibling.
+# After install, they can find each other if that subdirectory is an include
+# directory.
+
+# Add protocol buffer sources to an existing library target.
+# target:
+#     The name of the library target.
+# prefix:
+#     The install prefix for headers relative to `CMAKE_INSTALL_INCLUDEDIR`.
+#     This prefix should appear at the start of all your consumer includes.
+# ARGN:
+#     A list of .proto files.
+function(target_protobuf_sources target prefix)
+  set(dir "${CMAKE_CURRENT_BINARY_DIR}/pb-${target}")
+  file(MAKE_DIRECTORY "${dir}/${prefix}")
+
+  protobuf_generate(
+    TARGET ${target}
+    PROTOC_OUT_DIR "${dir}/${prefix}"
+    "${ARGN}"
+  )
+  target_include_directories(${target} SYSTEM PUBLIC
+    # Allows #include <package/path/to/file.proto> used by consumer files.
+    $<BUILD_INTERFACE:${dir}>
+    # Allows #include "path/to/file.proto" used by generated files.
+    $<BUILD_INTERFACE:${dir}/${prefix}>
+    # Allows #include <package/path/to/file.proto> used by consumer files.
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+    # Allows #include "path/to/file.proto" used by generated files.
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/${prefix}>
+  )
+  install(
+    DIRECTORY ${dir}/
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+    FILES_MATCHING PATTERN "*.h"
+  )
+endfunction()