correct handling of # in preprocessor

[guwirth]

• Check if # is the first token in the line, only then it is a preprocessor command. Otherwise it is a special character which is treated e.g. in macros like any other string.
• close sonar-scanner hangs with sonar-sxx 2.1 #2505

Sample:

BOOST_PP_EXPAND(#) ifndef BOOST_FT_config_valid
BOOST_PP_EXPAND(#)   define BOOST_FT_cc_id 1
BOOST_PP_EXPAND(#)   define BOOST_FT_cc_name implicit_cc
BOOST_PP_EXPAND(#)   define BOOST_FT_cc BOOST_PP_EMPTY
BOOST_PP_EXPAND(#)   define BOOST_FT_cond callable_builtin
#define _()
BOOST_PP_EXPAND(#)   include BOOST_FT_cc_file
#undef _
BOOST_PP_EXPAND(#)   undef BOOST_FT_cond
BOOST_PP_EXPAND(#)   undef BOOST_FT_cc_name
BOOST_PP_EXPAND(#)   undef BOOST_FT_cc
BOOST_PP_EXPAND(#)   undef BOOST_FT_cc_id
BOOST_PP_EXPAND(#) else
BOOST_PP_EXPAND(#)   undef BOOST_FT_config_valid
BOOST_PP_EXPAND(#) endif

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