cmMakefile: Let AddDefinition accept a value as cm::string_view

This changes `cmMakefile::AddDefinition` to take a `cm::string_view` as value
argument instead of a `const char *`.

Benefits are:
- `std::string` can be passed to `cmMakefile::AddDefinition` directly without
  the `c_str()` plus string length recomputation fallback.
- Lengths of literals passed to `cmMakefile::AddDefinition` can be computed at
  compile time.

In various sources uses of `cmMakefile::AddDefinition` are adapted to avoid
`std::string::c_str` calls and the `std::string` is passed directly.
Uses of `cmMakefile::AddDefinition`, where a `nullptr` `const char*` might
be passed to `cmMakefile::AddDefinition` are extended with `nullptr` checks.

1 files changed, 1 insertions, 2 deletions

diff --git a/Source/cmCMakePolicyCommand.cxx b/Source/cmCMakePolicyCommand.cxx
index 8da5ef7..ce046fc 100644
--- a/Source/cmCMakePolicyCommand.cxx
+++ b/Source/cmCMakePolicyCommand.cxx
@@ -209,8 +209,7 @@ bool cmCMakePolicyCommand::HandleGetWarningMode(
   }
 
   // Lookup the policy warning.
-  this->Makefile->AddDefinition(var,
-                                cmPolicies::GetPolicyWarning(pid).c_str());
+  this->Makefile->AddDefinition(var, cmPolicies::GetPolicyWarning(pid));
 
   return true;
 }