[Proj] Code style in Proj

[Kurt Schwehr]

Hi all,

I've been thinking about what is possible with the Proj code base with an
assumption that the code must be C89/C90 compatible.  I played around for a
few in godbolt with PJ_august.c (because it's small) and ended up with
this.  I tried to be aggressive as I could.  I think my modified version is
likely to be more static analyzer friendly.  What do you all think?   For
each of the change, is it better or worse for Proj as a whole?  My brain is
set to C++14/17, so I have a hard time evaluating this in context of what's
best for Proj in the long run.

Original: https://godbolt.org/g/ci5Wfo
Modified: https://godbolt.org/g/mz6NjD

Things I did:
- Combine definition and declaration
- Add const
- For double literals, have at least one digit before and after decimal
point.  e.g. .1 -> 0.1 and 3. -> 3.0
- Don't have assignments hidden inside expressions
- Convert #defines to typed const values
- IWYU - Include what you use... here math.h

Cheers,
-kurt

#include <math.h>

typedef struct { double x, y; }        XY;
typedef struct { double lam, phi; }    LP;
typedef struct PJconsts PJ;
XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
(void) P;
const double t = tan(0.5 * lp.phi);
const double c1 = sqrt(1.0 - t * t);
const double c = 1.0 + c1 * cos(lp.lam *= .5);
const double x1 = sin(lp.lam) *  c1 / c;
const double y1 =  t / c;
        const double x12 = x1 * x1;
        const double y12 = y1 *  y1;
        const double M = 1.333333333333333;
        const XY xy = {
            M * x1 * (3.0 + x12 - 3.0 * y12),
            M * y1 * (3.0 + 3.0 * x12 - y12)};
return xy;
}

Screenshot: https://www.flickr.com/photos/schwehr/39802222790
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