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

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.

Reported as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914

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

1 files changed, 6 insertions, 1 deletions

diff --git a/src/projects.h b/src/projects.h
index 2dc073fa..928985f6 100644
--- a/src/projects.h
+++ b/src/projects.h
@@ -458,7 +458,12 @@ struct PJconsts {
 struct ARG_list {
     paralist *next;
     char used;
-    char param[1];
+#ifdef __GNUC__
+    char param[0]; /* variable-length member */
+    /* Safer to use [0] for gcc. See https://github.com/OSGeo/proj.4/pull/1087 */
+#else
+    char param[1]; /* variable-length member */
+#endif
 };