cmake/clang: Disable -Wunused-but-set-variable warnings

Not sure why this is needed for this branch, but it pretty clearly is --
there are hundreds of set-but-unused variables in the codebase.

Signed-off-by: Keith Packard <keithp@keithp.com>

cmake/compiler/clang/compiler_flags.cmake

@@ -27,6 +27,7 @@ set_compiler_property(PROPERTY warning_base
                       -Wformat-security
                       -Wno-format-zero-length
                       -Wno-main
+                      -Wno-unused-but-set-variable
                       -Wno-typedef-redefinition
 )