[Infrastructure] More fixes to precompiled headers (#17524)

Author: ktf

CMakeLists.txt

@@ -9,7 +9,10 @@
 # granted to it by virtue of its status as an Intergovernmental Organization
 # or submit itself to any jurisdiction.
 
-cmake_minimum_required(VERSION 3.23 FATAL_ERROR)
+# 3.27 for $<COMPILE_ONLY:>, which O2PhysicsAddWorkflow.cmake uses to give a
+# workflow the precompiled header carrier's compile usage requirements without
+# also putting it on the link line. Builds use CMake 4.1 (alidist cmake.sh).
+cmake_minimum_required(VERSION 3.27 FATAL_ERROR)
 
 project(O2Physics
         VERSION 0.0.1

cmake/O2PhysicsAddWorkflow.cmake

@@ -69,11 +69,25 @@ function(o2physics_add_dpl_workflow baseTargetName)
     #   cmake_pch.hxx.gch: not used because `RANS_ENABLE_JSON' not defined
     # The carrier links O2Physics::AnalysisCore, which reaches O2::rANS and its
     # INTERFACE -DRANS_ENABLE_JSON, while a workflow that links only
-    # O2::Framework (the converters, the tutorials) never sees it. Hand every
-    # consumer the carrier's definitions so the two agree. Definitions only --
-    # this must not add link dependencies to targets that do not want them.
-    target_compile_definitions(${targetExeName} PRIVATE
-                               $<TARGET_PROPERTY:${_pch},COMPILE_DEFINITIONS>)
+    # O2::Framework (the converters, the tutorials) never sees it.
+    #
+    # Copying COMPILE_DEFINITIONS fixed that case and revealed another: CI then
+    # failed on `_REENTRANT' not defined, on a compile line that DID carry
+    # -DRANS_ENABLE_JSON -- so the copy was working and simply does not reach
+    # far enough. Whatever supplies _REENTRANT arrives at the carrier as
+    # something other than a compile definition (-pthread, which travels in
+    # INTERFACE_COMPILE_OPTIONS, is the likely route), so adding options alone
+    # would only move the goalposts to whichever kind of usage requirement goes
+    # missing next.
+    #
+    # $<COMPILE_ONLY:> applies a target's *compile* usage requirements --
+    # definitions, options, include directories, features -- without placing it
+    # on the link line or creating a link dependency, which is the constraint
+    # that ruled out simply linking the carrier. It transfers the whole
+    # preprocessor state the carrier compiled with, and that state is exactly
+    # what GCC compares, rather than one property of it at a time.
+    # Requires CMake >= 3.27.
+    target_link_libraries(${targetExeName} PRIVATE $<COMPILE_ONLY:${_pch}>)
     target_precompile_headers(${targetExeName} REUSE_FROM ${_pch})
   endif()