Remove PROJ_COMPILATION=1 definition

[mwtoews]

These do not appear to be used, and I can't see what they are for. They were added from the d928db1 mega commit for the "GDAL SRS barn" work.

@rouault is it OK to remove these?

[rouault]

is it OK to remove these?

yes, can't remember why this was added and doesn't appear to be used anywhere

[cffk]

I can see a use for PROJ_COMPILATION...

I'd like to have geodesic.[ch] usable independently of PROJ (it's
currently a separate mini-project within GeographicLib). In that case
it would be nice if the compiler could know if geodesic.c were being
compiled as part of PROJ (in which case proj_math.h can be included) or
not. Currently this uses the PJ_LIB__ macro, but I don't think this
ever worked. However PROJ_COMPILATION would fill the bill.

[mwtoews]

@cffk sure, we can keep it.

Are there any other use-cases for this define? If not, then with the CMake setup, I may just reduce the scope of this define to lib_proj.cmake via:

# Use PROJ_COMPILATION to distinguish code compiled as part of PROJ 
set_source_files_properties(
  geodesic.c geodesic.h
  PROPERTIES COMPILE_DEFINITIONS PROJ_COMPILATION
)

And with the autotools setup, just set it in AM_CPPFLAGS, but it'd still be global.

[cffk]

Since it'll be a few months before I'm in a position to update the geodesic
routines, why don't you remove the definition for now. I'll add it back in
when I need it (narrowing the scope if I can).

[mwtoews]

@cffk great to know about new development with the C geodesic routines! And yup, it should be simple to re-apply this macro to autoconf/cmake when the time comes.