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, 4 insertions, 2 deletions

diff --git a/Source/cmCPluginAPI.cxx b/Source/cmCPluginAPI.cxx
index 8c2d987..80ca898 100644
--- a/Source/cmCPluginAPI.cxx
+++ b/Source/cmCPluginAPI.cxx
@@ -65,8 +65,10 @@ unsigned int CCONV cmGetMinorVersion(void*)
 
 void CCONV cmAddDefinition(void* arg, const char* name, const char* value)
 {
-  cmMakefile* mf = static_cast<cmMakefile*>(arg);
-  mf->AddDefinition(name, value);
+  if (value) {
+    cmMakefile* mf = static_cast<cmMakefile*>(arg);
+    mf->AddDefinition(name, value);
+  }
 }
 
 /* Add a definition to this makefile and the global cmake cache. */