[Bug c/78829] bit-rotten "C99 mode" references in GCC manual

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78829

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Note that some of these (e.g. VLAs) are extensions in C++ as well.  And 
some include features beyond the standard ones (e.g. _Complex is standard 
C99, but the syntax for complex constants, and __real__ and __imag__ 
(which are also lvalues) and ~ for complex conjugation, are all 
extensions, so still need documenting as such).