cmake: don't let a stale cache poison the ROOT7 API compile check.

check_cxx_source_compiles() caches MUSRVIEW_HAS_ROOT7_API under a plain
variable name with no dependency tracking, so a failure recorded once
(e.g. against an older ROOT built into the same build/ directory) kept
being reported as missing on every later reconfigure even after ROOT was
fine. Unset both cache entries before the check so it always re-runs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmake/MusrfitCheckROOT7.cmake

@@ -138,6 +138,15 @@ set(CMAKE_REQUIRED_QUIET TRUE)
 set(_musrview_try_compile_type_save "${CMAKE_TRY_COMPILE_TARGET_TYPE}")
 set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
 
+#--- check_cxx_source_compiles() caches its result under a plain variable
+#--- name with no dependency tracking: if it once failed (e.g. against an
+#--- older ROOT in this same build directory) it would keep reporting that
+#--- stale failure on every later reconfigure, even after ROOT got fixed,
+#--- since neither the test source nor ROOT_VERSION are part of the cache
+#--- key. Unset both results first so the compile check always re-runs. ------
+unset(MUSRVIEW_HAS_ROOT7_API CACHE)
+unset(MUSRVIEW_HAS_ROOT7_API_EXPERIMENTAL CACHE)
+
 set(MUSRVIEW_NS "ROOT::")
 string(CONFIGURE "${MUSRVIEW_ROOT7_API_TEST_SRC}" _musrview_api_src @ONLY)
 check_cxx_source_compiles("${_musrview_api_src}" MUSRVIEW_HAS_ROOT7_API)