syntax error caused by unconditional `__has_include` usage

[firewave]

Encountered while working on #438.

#if __has_include(<sys/single_threaded.h>)
# include <sys/single_threaded.h>
#endif

a.cpp:1: syntax error: failed to evaluate #if condition, division/modulo by zero

[firewave]

It works fine when specifying -std=c++17.

I think we should default to the latest standard by default akin to Cppcheck.

[ludviggunne]

I believe GCC supports __has_include for all standards, so we could also check if the standards starts with gnu....

[ludviggunne]

I opened #452 which I think should give a more reasonable error message when __has_include is not defined.

[ludviggunne]

I believe GCC supports __has_include for all standards, so we could also check if the standards starts with gnu....

#454 addresses this

[firewave]

It now reports:

a.cpp:1: syntax error: failed to evaluate #if condition, undefined function-like macro invocation: __has_include( ... )