Fix wrong behaviour of torad_coord() with gcc 8.1 -O2 (fixes #1084)

[rouault]

torad_coord() of gie.c has this sequence:

if( cond )
    axis = l->param + strlen ("axis=");
n = strlen (axis);

When the if branch is evaluated, n is always zero
even if on inspection axis is non empty

The reason is that the struct ARG_list which is the
type of l use a variable-length array for the param member

struct ARG_list {
paralist *next;
char used;
char param[1];
};

But this is not a proper way of declaring it, and
gcc 8 has apparently optimizations to detect that l->param + 5
points out of the array, hence it optimizes strlen() to 0.

According to https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html,
the proper way of declaring such arrays is to use [0]

[rouault]

looking closer, looks like a too aggressive optimization in that particular case. Filed as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914

[kbevers]

Do you still want to merge this pull request or just wait until the GCC guys sort their things out?

[rouault]

I think that it is reasonable to merge it, even if it is a GCC bug. This version is out and so even if it is fixed, there's always the risk someone sticking with this version and hitting the issue. After all the use of one-element array was discouraged by the gcc docs, so the use of zero-length is a better practice.
(an even better practice would be to just avoid the use of such tricky features)

[kbevers]

Alright, let's merge it then. Perhaps put in a comment explaining what's going on.

After 5.2 is released we can finally begin to make breaking changes in projects.h. We should use that opportunity to deal with this in a smarter way that doesn't require hacks for different compilers.