For clang compiler the compiler's flag -Wno-unused-but-set-variable
was set based on compiler version. This approach could result in
false positive detection for presence of compiler option since
only first three groups of digits in compiler version taken into account
and it could lead to inaccuracy in determining of supported compiler's
features.

Correct way to detect options supported by a compiler is to use
the macros  MY_CHECK_CXX_COMPILER_FLAG and to check the result of
variable with prefix have_CXX__
So, to check whether compiler does support the option
 -Wno-unused-but-set-variable
the macros
 MY_CHECK_CXX_COMPILER_FLAG(-Wno-unused-but-set-variable)
should be called and the result variable
 have_CXX__Wno_unused_but_set_variable
be tested for assigned value.