cmake: fix USE_SYSTEM_BOOST behavior ... (#6837)

CMakeLists.txt

@@ -341,13 +341,6 @@ function(get_timestamp _var)
     set(${_var} "${timestamp}" PARENT_SCOPE)
 endfunction()
 
-# Prevent boost from linking against libs when building
-add_definitions(-DBOOST_ERROR_CODE_HEADER_ONLY
-    -DBOOST_SYSTEM_NO_LIB
-    -DBOOST_DATE_TIME_NO_LIB
-    -DBOOST_REGEX_NO_LIB
-)
-
 # generate git/build information
 include(GetGitRevisionDescription)
 get_git_head_revision(GIT_REF_SPEC GIT_REV)
@@ -355,17 +348,23 @@ git_describe(GIT_DESC --always --long --dirty)
 git_branch_name(GIT_BRANCH)
 get_timestamp(BUILD_DATE)
 
-if (NOT USE_SYSTEM_BOOST)
-    add_definitions( -DBOOST_ALL_NO_LIB )
+# Boost
+# Prevent boost from linking against libs when building
+add_definitions(-DBOOST_ERROR_CODE_HEADER_ONLY
+    -DBOOST_SYSTEM_NO_LIB
+    -DBOOST_DATE_TIME_NO_LIB
+    -DBOOST_REGEX_NO_LIB
+)
+if (USE_SYSTEM_BOOST)
+    find_package(Boost 1.70.0 COMPONENTS container locale serialization iostreams REQUIRED)
 endif()
 
 enable_testing()
 add_subdirectory(externals)
 
-# Boost
-if (USE_SYSTEM_BOOST)
-    find_package(Boost 1.70.0 COMPONENTS serialization iostreams REQUIRED)
-else()
+# Boost (bundled)
+if (NOT USE_SYSTEM_BOOST)
+    add_definitions( -DBOOST_ALL_NO_LIB )
     add_library(Boost::boost ALIAS boost)
     add_library(Boost::serialization ALIAS boost_serialization)
     add_library(Boost::iostreams ALIAS boost_iostreams)

externals/CMakeLists.txt

@@ -12,6 +12,8 @@ include(DownloadExternals)
 include(ExternalProject)
 
 # Boost
+if (NOT USE_SYSTEM_BOOST)
+    message(STATUS "Including vendored Boost library")
     set(BOOST_ROOT "${CMAKE_SOURCE_DIR}/externals/boost" CACHE STRING "")
     set(Boost_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/externals/boost" CACHE STRING "")
     set(Boost_NO_SYSTEM_PATHS ON CACHE BOOL "")
@@ -31,8 +33,8 @@ add_library(
         ${CMAKE_SOURCE_DIR}/externals/boost/libs/iostreams/src/mapped_file.cpp
     )
     target_link_libraries(boost_iostreams PUBLIC boost)
-
 # Add additional boost libs here; remember to ALIAS them in the root CMakeLists!
+endif()
 
 # Catch2
 set(CATCH_INSTALL_DOCS OFF CACHE BOOL "")