Compile with explicit language flag when source LANGUAGE property is set

This change was originally made by commit 74b1c9fc8e (Explicitly specify
language flag when source LANGUAGE property is set, 2020-06-01,
v3.19.0-rc1~722^2), but it was reverted by commit 30aa715fac (Revert
"specify language flag when source LANGUAGE property is set",
2020-11-19) to restore compatibility with pre-3.19 behavior.

Implement the change again, but add policy CMP0119 to make this change
while preserving compatibility with existing projects.

Note that the `Compiler/{Clang,Intel,MSVC}-CXX` modules do not need to
specify `-TP` for their MSVC-like variants because we already use the
flag in `CMAKE_CXX_COMPILE_OBJECT`.  Similarly for `Compiler/XL-CXX`
and `Platform/Windows-Embarcadero`.

Note also that this does not seem possible to implement for XL C.
Even with `-qsourcetype=c`, `xlc` complains about an unknown suffix:
`1501-218 (W) file /.../AltExtC.zzz contains an incorrect file suffix`.
It returns non-zero even with `-qsuppress=1501-218`.

Co-Author: Robert Maynard <robert.maynard@kitware.com>
Fixes: #14516, #20716

1 files changed, 9 insertions, 1 deletions

diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index 4ac08e0..86cbf80 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -183,7 +183,15 @@ std::string cmNinjaTargetGenerator::ComputeFlagsForObject(
     }
   }
 
-  std::string flags = this->GetFlags(language, config, filterArch);
+  std::string flags;
+  // Explicitly add the explicit language flag before any other flag
+  // so user flags can override it.
+  this->GeneratorTarget->AddExplicitLanguageFlags(flags, *source);
+
+  if (!flags.empty()) {
+    flags += " ";
+  }
+  flags += this->GetFlags(language, config, filterArch);
 
   // Add Fortran format flags.
   if (language == "Fortran") {