build: Improve error message about mismatching git tags (#5205)

This message is triggered by the check that the tag of an auto-build
dependency matches the SHA hash we use as a cross-check. This patch is
just amending the error message to reveal the way to override the check,
so people (like me) don't have to go hunting for it if they are
purposely trying to build from a nonstandard commit of a dependency.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/cmake/dependency_utils.cmake

@@ -680,7 +680,8 @@ macro (build_dependency_with_cmake pkgname)
             message (FATAL_ERROR
                 "${pkgname}: Tag ${_pkg_GIT_TAG} resolved to commit "
                 "${_pkg_actual_commit}, but expected ${_pkg_GIT_COMMIT}. "
-                "This may indicate the tag was tampered with or moved.")
+                "This may indicate the tag was tampered with or moved. "
+                "Setting ${PROJECT_NAME}_DEPENDENCY_BUILD_ALLOW_UNVERIFIED_TAGS=ON will disable this check.")
         endif ()
     elseif (NOT "${_pkg_GIT_COMMIT}" STREQUAL "")
         # Only commit hash specified: checkout that commit directly.