build: use find_dependency() instead find_package() in config file

SeastarDependencies.cmake is used in two scenarios:

- configure Seastar library, when building Seastar itself.
- configure Seastar library, when a parent project tries to
  detect Seastar library. when building the Seastar application.

in the second case, we should respect the "REQUIRED" and "QUIET"
options passed to `find_package(Seastar)`. and CMake provides
`find_dependency()` macro for this purpose.

in this change, we conditionally call `find_dependency()` when
`SeastarDependencies.cmake` is used when this script is called
when finding Seastar library. this provides a better developer
experience to Seastar developers.

Signed-off-by: Kefu Chai <[email protected]>

cmake/SeastarConfig.cmake.in

@@ -20,9 +20,6 @@
 # Copyright (C) 2018 Scylladb, Ltd.
 #
 
-# We would like to use `find_dependency`, but it is not supported properly until CMake 3.8.
-#include (FindDependencyMacro)
-
 list (APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR})
 
 if (CMAKE_CXX_STANDARD)

cmake/SeastarDependencies.cmake

@@ -20,6 +20,8 @@
 # Copyright (C) 2019 Scylladb, Ltd.
 #
 
+include(CMakeParseArguments)
+
 #
 # CMake bundles `FindBoost.cmake`, which is coupled to the Boost version. If
 # we're on a system without a recent enough version of `FindBoost.cmake`, then
@@ -48,9 +50,26 @@ if (Boost_VERSION_STRING VERSION_LESS 1.81.0)
     INTERFACE_COMPILE_DEFINITIONS "BOOST_NO_CXX98_FUNCTION_BASE")
 endif ()
 
-macro (seastar_find_dep package)
-  find_package (${package} ${ARGN})
-endmacro ()
+if (CMAKE_FIND_PACKAGE_NAME)
+  # used inside find_package(Seastar)
+  include (CMakeFindDependencyMacro)
+
+  macro (seastar_find_dep package)
+    cmake_parse_arguments(args "REQUIRED" "" "" ${ARGN})
+    if (arg_REQUIRED)
+      find_dependency (${package} ${arg_UNPARSED_ARGUMENTS})
+    else ()
+      # some packages are not REQUIRED, so we just check for them instead of
+      # populating "REQUIRED" from the original find_package() call.
+      find_package (${package} ${ARGN})
+    endif ()
+  endmacro ()
+else()
+  macro (seastar_find_dep package)
+    # used when configuring Seastar
+    find_package (${package} ${ARGN})
+  endmacro ()
+endif ()
 
 macro (seastar_find_dependencies)
   #