Fix x86 idiv with (negative) 4-byte ints
Fix constant division/modulo with negative integers
Fix finding an invalid trigraph sequence
Correct parsing of hex-floating constants like 0x.[0-9]...
Add -Wgnu
Warn on GNU address-of-label
Disable -Wgnu by default
cpp: fix extra "warning:" message for redef
cpp handles -Wall and -Weverything
cpp errors on redefinition of builtins (__TIME__, etc)
cpp handles backslash-newline with spaces between
Handle CHECK directives with multiple ^ chars
cpp warns on backslash-newline with spaces between
Handle floating point expressions in if / ?:
Case expressions are "constant", i.e. no-comma