Log Message:
Update devel/c++gsl to version 4.0.0

Pkgsrc changes:
Use devel/cmake/build.mk
CMake only looks for the c++ compiler so remove c
from USE_LANGAGES.
Due to a breaking change, that gsl/multi_span is not
installed anymore and gsl::narrow is in gsl/narrow not
gsl/util, update BUILDLINK_API_DEPENDS.
Change maintainer, ok'ed by minskim.

Changes from Release Notes:

* Deprecation of gsl::string_span
* Removal of <gsl/multi_span>
* Header files dropped the gsl_ prefix
* Changes to not_null
* gsl::span and std::span now use the correct specialization of gsl::at
* The zstring family no longer requires empty brackets to be used
  - for example, void foo(zstring<> str); should now be
    void foo(zstring str);
* gsl::narrowing_error now has a helpful what() message
* finally and final_action are now [[nodiscard]]
* GSL will work in environments where exceptions are disabled, with
  some caveats
* GSL will work in environments which do not support ios, via the
  addition of the GSL_NO_IOSTREAMS flag
* CMake and build improvements