Cephes Updates

[varunagrawal]

• Add Cephes README and License
• Add dllexport for cross-platform extern support.

[ProfFan]

Hi Varun, per the discussions in #1605 and (recent Windows-related issue), can we just make this header all inline? These symbols does not need to be exported if they are only used internally, and we save one extra dynamic library.

[ProfFan]

Some of the source files are licensed with the Boost license, for example, here:

/*
 * (C) Copyright John Maddock 2006.
 * Use, modification and distribution are subject to the
 * Boost Software License, Version 1.0. (See accompanying file
 *  LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 */

in igami.c that we are using. Could you add this to the LICENSE.txt?

[varunagrawal]

Some of the source files are licensed with the Boost license, for example, here:

/*
 * (C) Copyright John Maddock 2006.
 * Use, modification and distribution are subject to the
 * Boost Software License, Version 1.0. (See accompanying file
 *  LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 */

in igami.c that we are using. Could you add this to the LICENSE.txt?

I think this is fine and doesn't need to be added to the LICENSE.txt file since the Boost license states that only the license header should be added to files using Boost code (which is already done). I'll go ahead and merge this PR.

[zzodo]

I faced a problem with this dllexport.h when building my own c++ library along with system-installed GTSAM on default branch.

/usr/local/include/gtsam/3rdparty/cephes/cephes.h:4:10: fatal error: dllexport.h: No such file or directory
    4 | #include "dllexport.h"
      |          ^~~~~~~~~~~~~
compilation terminated.

This compilation error was fixed by adding dllexport.h to gtsam/3rdparty/cephes/CMakeLists.txt, but I am not sure this is what it should be.
@varunagrawal Can you confirm this?

EDIT:
e.g.

set(CEPHES_HEADER_FILES
    cephes.h
    cephes/dd_idefs.h
    cephes/dd_real.h
    cephes/dd_real_idefs.h
    cephes/expn.h
    cephes/igam.h
    cephes/lanczos.h
    cephes/mconf.h
    cephes/polevl.h
    cephes/sf_error.h
    dllexport.h) # add here

[ProfFan]

@zzodo Could you put up a quick PR?

[zzodo]

@ProfFan Sure, please check #1748.