Complements f2b3604

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...

We are relying more and more on C99 math functions. On C89 systems where
those functions are not available our own custom versions of those
functions are used instead. So far these has been spread across the code
base. This commit gathers them in the same file and introduces the
proj_math.h header. The build system checks for C99 math functions. If
not found the proj_math.h header make sure that C99 functions are
defined as their pj_ equivalent.

Ideally proj_math.h is included instead of math.h. This removes the need
for any checks against HAVE_C99_MATH in the code making it easier to
read.

For this commit the functions hypot, log1p and asinh has been taken care
of.

This uses a linear approximation of tan(x+pi/4) for better precision at small latitudes.
As a result points of latitude 0 maintain a 0 Y coordinate and 0,0 is transformed to 0,0

* Avoid using angle brackets for PROJ headers.

Switching from #include <proj.h> to #include "proj.h",
and correspondingly for projects.h and proj_api.h,
reduces the risk of accidentally picking up a system
installed older version of the header while building
a new version of PROJ.

* Also handle geodesic.h