Merge pull request #173 from ValeevGroup/evaleev/cmake/minimal-boost-…

…version

[cmake] insist on Boost 1.71 or later, if discovering

external/boost.cmake

@@ -1,5 +1,18 @@
 # -*- mode: cmake -*-
 
+# update the Boost version that we can tolerate
+if (NOT DEFINED Boost_OLDEST_BOOST_VERSION)
+  set(Boost_OLDEST_BOOST_VERSION ${BTAS_OLDEST_BOOST_VERSION})
+else()
+  if (${Boost_OLDEST_BOOST_VERSION} VERSION_LESS ${BTAS_OLDEST_BOOST_VERSION})
+    if (DEFINED CACHE{Boost_OLDEST_BOOST_VERSION})
+      set(Boost_OLDEST_BOOST_VERSION "${BTAS_OLDEST_BOOST_VERSION}" CACHE STRING "Oldest Boost version to use" FORCE)
+    else()
+      set(Boost_OLDEST_BOOST_VERSION ${BTAS_OLDEST_BOOST_VERSION})
+    endif()
+  endif()
+endif()
+
 # Boost can be discovered by every (sub)package but only the top package can *build* it ...
 # in either case must declare the components used by BTAS
 set(required_components

external/versions.cmake

@@ -1,7 +1,7 @@
 set(BTAS_TRACKED_VGCMAKEKIT_TAG d5c0a6f9ff6dc97cbb5132912733e1eb1cf73f1e)
 
-# likely can use earlier, but
+# oldest Boost we can tolerate ... likely can use an earlier version, but:
 # - as of oct 2023 tested with 1.71 and up only
 # - avoids the need to avoid 1.70 in which Boost.Container is broken
 # - matches the version provided by https://github.com/Orphis/boost-cmake as of oct 2023
-set(BTAS_TRACKED_BOOST_VERSION 1.71)
+set(BTAS_OLDEST_BOOST_VERSION 1.71)