This is intended to supersed https://github.com/OSGeo/proj.4/pull/1080
with a number of differences.

What is kept from #1080 is not forcing the ellipsoid_params to be the one
of a sphere. This is not required for correct coordinate computation and
avoid lying on the various distorsion parameters.

For better interoperability with EPSG, we also no longer force the
lam0 parameter to 0, because
https://www.epsg-registry.org/export.htm?gml=urn:ogc:def:method:EPSG::1024
has a provision for it, even if in practice they will always be zero
phi0 should always be zero and is not used by the formulas.

Another difference with the #1080 approach is that we do not force the
WGS84 ellipsoid. Perhaps someone will use webmerc for another planet,
even if that is a crazy idea...

Implement the Equal Earth projection (closes #1085) 

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]

src/pj_strerrno.c:96:20: runtime error: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself
    #0 in pj_strerrno proj/src/pj_strerrno.c:96:20
    #1 in (anonymous namespace)::ProjErrnoStringTest_ProjErrnos_Test::TestBody() test/unit/proj_errno_string_test.cpp:47:5

ASAN UndefinedBehaviorSanitizer: signed-integer-overflow

Issue revealed by proj_errno_string_test.cpp add in
https://github.com/OSGeo/proj.4/commit/b87b59106879188ffc684a41a9de638ac5fd02bf