build: find Boost in installed header trees too

The shim only probed <root>/boost/version.hpp, so a BOOST_ROOT pointing at
an installed tree (<root>/include/boost/version.hpp) was rejected even
though the headers were there. Probe with find_path, which covers both
layouts and the default search paths, and report through
find_package_handle_standard_args so the requested version is honoured.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jcelerier

cmake/FindBoost.cmake

@@ -2,8 +2,10 @@
 #
 # Avendish only needs Boost headers (Boost::boost / Boost::headers). Strategy:
 #  1. defer to Boost's own BoostConfig.cmake when it is installed;
-#  2. otherwise, satisfy the request from a plain header tree given via
-#     BOOST_ROOT / Boost_INCLUDE_DIR.
+#  2. otherwise, satisfy the request from a plain header tree, accepting both
+#     the source layout (<root>/boost/version.hpp) and the installed one
+#     (<root>/include/boost/version.hpp), and falling back to the default
+#     search paths.
 
 # 1. Prefer config mode if available.
 find_package(Boost ${Boost_FIND_VERSION} CONFIG QUIET)
@@ -13,22 +15,36 @@ endif()
 
 # 2. Header-only fallback.
 if(NOT Boost_INCLUDE_DIR)
-  if(BOOST_ROOT)
-    set(Boost_INCLUDE_DIR "${BOOST_ROOT}")
-  elseif(DEFINED ENV{BOOST_ROOT})
-    set(Boost_INCLUDE_DIR "$ENV{BOOST_ROOT}")
-  endif()
+  set(_avnd_boost_hints "")
+  foreach(_avnd_boost_root
+          "${BOOST_ROOT}" "$ENV{BOOST_ROOT}"
+          "${Boost_ROOT}" "$ENV{Boost_ROOT}"
+          "${BOOSTROOT}" "$ENV{BOOSTROOT}")
+    if(_avnd_boost_root)
+      list(APPEND _avnd_boost_hints "${_avnd_boost_root}")
+    endif()
+  endforeach()
+
+  # PATH_SUFFIXES covers installed trees (<root>/include/boost/version.hpp);
+  # the bare hint covers unpacked source trees (<root>/boost/version.hpp).
+  find_path(Boost_INCLUDE_DIR
+    NAMES boost/version.hpp
+    HINTS ${_avnd_boost_hints}
+    PATH_SUFFIXES include)
+  unset(_avnd_boost_hints)
+  unset(_avnd_boost_root)
 endif()
 
 if(Boost_INCLUDE_DIR AND EXISTS "${Boost_INCLUDE_DIR}/boost/version.hpp")
-  file(STRINGS "${Boost_INCLUDE_DIR}/boost/version.hpp" _boost_ver_line
+  file(STRINGS "${Boost_INCLUDE_DIR}/boost/version.hpp" _avnd_boost_ver_line
        REGEX "#define BOOST_LIB_VERSION ")
-  string(REGEX MATCH "\"([0-9]+)_([0-9]+)\"" _ "${_boost_ver_line}")
+  string(REGEX MATCH "\"([0-9]+)_([0-9]+)" _ "${_avnd_boost_ver_line}")
   set(Boost_VERSION_MAJOR "${CMAKE_MATCH_1}")
   set(Boost_VERSION_MINOR "${CMAKE_MATCH_2}")
+  set(Boost_VERSION_PATCH 0)
   set(Boost_VERSION_STRING "${CMAKE_MATCH_1}.${CMAKE_MATCH_2}.0")
   set(Boost_VERSION "${Boost_VERSION_STRING}")
-  set(Boost_FOUND TRUE)
+  unset(_avnd_boost_ver_line)
 
   if(NOT TARGET Boost::boost)
     add_library(Boost::boost INTERFACE IMPORTED GLOBAL)
@@ -40,11 +56,12 @@ if(Boost_INCLUDE_DIR AND EXISTS "${Boost_INCLUDE_DIR}/boost/version.hpp")
     set_target_properties(Boost::headers PROPERTIES
       INTERFACE_INCLUDE_DIRECTORIES "${Boost_INCLUDE_DIR}")
   endif()
-else()
-  set(Boost_FOUND FALSE)
-  if(Boost_FIND_REQUIRED)
-    message(FATAL_ERROR
-      "FindBoost shim: Boost not found. Install Boost with CMake config "
-      "files, or pass -DBOOST_ROOT=<path to a Boost header tree>.")
-  endif()
 endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Boost
+  REQUIRED_VARS Boost_INCLUDE_DIR
+  VERSION_VAR Boost_VERSION_STRING
+  FAIL_MESSAGE
+    "Boost headers not found. Install Boost with CMake config files, or pass \
+-DBOOST_ROOT=<path to a Boost tree>.")