gtypes: Define signed integer minimum values in terms of maximum values
This is what limits.h does. It avoids a problem when compiling with MSVC (in C++ mode) on Windows, whereby the 0x80000000 constant is interpreted as unsigned, and then the unary minus is applied to it.
Add a test which should hopefully catch problems with different compilers’ interpretations of this in future.
Signed-off-by: Philip Withnall firstname.lastname@example.org
Closes #1663 (closed)