Remove HAVE_MPROTECT cmake check

Summary

The posix function mprotect() is defined in the header sys/mman.h. Instead of implementing a cmake check for the header and whether a c source file compiles, this MR uses __has_include(<sys/mman.h>) instead.

Note, the cmake variable HAVE_MPROTECT is not defined anymore. Also the config.h variables HAVE_MPROTECT and HAVE_SYS_MMAN_H are not defined anymore. They are used exclusively inside debugallocator.hh and thus (for backwards compatibility) defined there if the corresponding header is found.

CHANGELOG.md

 # Master (will become release 2.8)
 
+- Remove the cmake check for `HAVE_MPROTECT` and also do not define this variable in the
+  `config.h` file. It is defined only inside the header `debugallocator.hh`.
+
 - Deprecate fallback implementations `Dune::Std::apply`, `Dune::Std::bool_constant`, and
   `Dune::Std::make_array` in favor of std c++ implementations.