adds support for optional args, with backwards compatibility

[dmalan]

This changes get_* to expect a string as their sole parameter, which is then used to prompt (and, if need be, re-prompt) the user for the intended type of input. Our prior use of a Retry: prompt was feeling arbitrary. Plus, this way, students can control not only the first prompt a user sees but any (identical) re-prompts.

Maintains backwards compatibility with prior versions of the library such that students (for now) can still call Get* and get_* with no arguments; in those cases, the library behaves as it used to with Retry: re-prompts.

After CS50 AP 1617 wraps in June 2017, we can remove support for Get* altogether (or just leave the deprecation warnings in there for a year). After CS50 AP 1718 wraps in June 2018, we can remove support for optional arguments to get_*, instead requiring that a user provide a prompt (which could still be ""); probably no need at that point to allow NULL as input, since that's only used now as a sentinel value to indicate that a caller is using the functions' older argument-less calling conventions.

[dmalan]

Per #50, not yet clear how best to handle the std=c99 issue, if not via std=gnu99.

[dmalan]

Added macro checks for GNU extensions.

[dmalan]

@kzidane, @crossroads1112 able to test on Visual C?

[dmalan]

[cmlsharp]

Sorry for my being late, but as an alternative, what about temporarily (until optional arguments are no longer required) doing something like the following:

In cs50.h:

struct OptionalPrompt {
    char _sentinel; // Avoid __VA_ARGS__ issues
    string prompt;
}

char get_char_prompt(struct OptionalPrompt *opt_prompt);
#define get_char(...) get_char_prompt(&(struct OptionalPrompt) {._sentinel = 0, __VA_ARGS__})

In cs50.c:

char get_char_prompt(struct OptionalPrompt *opt_prompt) 
{
    string prompt = opt_prompt->prompt;
    // The rest of get_char's implementation
}

In main.c:

int main(void) 
{
    /* expands to get_char_prompt(&(struct OptionalPrompt) { .sentinel = 0, }) which is completely valid
     * opt_arg->prompt is implicitly set to 0 (NULL) since it isn't specified
    */
    char c1 = get_char(); 

    /* expands to get_char_prompt(&(struct OptionalPrompt) { .sentinel = 0, "Enter a character" }) */
    char c2 = get_char("Enter a character: ");
}

Pros: Completely standard ISO C (no need to worry about VC vs GNUC or change the Makefile)
Cons: It does mean that the functions in cs50.c will not actually be named get_char but rather some similar name like get_char_prompt. This does add some complexity but I'd argue significantly less than the preprocessor solution currently in cs50.h.

[dmalan]

@crossroads1112, I worry that introducing the *_prompt variants is wont to cause confusion (or at least curiosity) during debugging, when the symbols might appear among stack frames. Better, I think, to hide the backwards compatibility with some cleverness for just a year, especially since we're relying on extensions for garbage collection anyway.

If this proves an issue for, e.g., Windows, we can just offer a version 9.0.0 branch in parallel with support only for the required-argument functions.