Makefiles: Do not use '\#' escape sequence with Windows-style make tools

Since commit fbf7a92975 (Makefile: Handle '#' in COMPILE_OPTIONS,
2014-08-12, v3.1.0-rc1~174^2) we escape `#` as `\#` in `flags.make`
variable assignments so that they are not treated as a comment.
Windows-style make tools like NMake do not interpret backslashes
in that way.  Other means will be needed to handle `#` in contexts
where it is even possible.  The test suite is not covering this
for NMake anyway, and actually has a workaround in `Tests/TryCompile`
for the old behavior, which we can now update.

Source/cmGlobalBorlandMakefileGenerator.h

@@ -46,6 +46,7 @@ public:
 
   bool AllowNotParallel() const override { return false; }
   bool AllowDeleteOnError() const override { return false; }
+  bool CanEscapeOctothorpe() const override { return true; }
 
 protected:
   std::vector<GeneratedMakeCommand> GenerateBuildCommand(

Source/cmGlobalUnixMakefileGenerator3.cxx

@@ -117,6 +117,12 @@ void cmGlobalUnixMakefileGenerator3::ComputeTargetObjectDirectory(
   gt->ObjectDirectory = dir;
 }
 
+bool cmGlobalUnixMakefileGenerator3::CanEscapeOctothorpe() const
+{
+  // Make tools that use UNIX-style '/' paths also support '\' escaping.
+  return this->ForceUnixPaths;
+}
+
 void cmGlobalUnixMakefileGenerator3::Configure()
 {
   // Initialize CMAKE_EDIT_COMMAND cache entry.

Source/cmGlobalUnixMakefileGenerator3.h

@@ -157,6 +157,9 @@ public:
   /** Does the make tool tolerate .DELETE_ON_ERROR? */
   virtual bool AllowDeleteOnError() const { return true; }
 
+  /** Does the make tool interpret '\#' as '#'?  */
+  virtual bool CanEscapeOctothorpe() const;
+
   bool IsIPOSupported() const override { return true; }
 
   void ComputeTargetObjectDirectory(cmGeneratorTarget* gt) const override;

Source/cmMakefileTargetGenerator.cxx

@@ -341,12 +341,16 @@ void cmMakefileTargetGenerator::WriteTargetLanguageFlags()
                           << "\n";
   }
 
+  bool const escapeOctothorpe = this->GlobalGenerator->CanEscapeOctothorpe();
+
   for (std::string const& language : languages) {
     std::string defines = this->GetDefines(language, this->GetConfigName());
     std::string includes = this->GetIncludes(language, this->GetConfigName());
+    if (escapeOctothorpe) {
       // Escape comment characters so they do not terminate assignment.
       cmSystemTools::ReplaceString(defines, "#", "\\#");
       cmSystemTools::ReplaceString(includes, "#", "\\#");
+    }
     *this->FlagFileStream << language << "_DEFINES = " << defines << "\n\n";
     *this->FlagFileStream << language << "_INCLUDES = " << includes << "\n\n";
 
@@ -357,7 +361,9 @@ void cmMakefileTargetGenerator::WriteTargetLanguageFlags()
     for (const std::string& arch : architectures) {
       std::string flags =
         this->GetFlags(language, this->GetConfigName(), arch);
+      if (escapeOctothorpe) {
         cmSystemTools::ReplaceString(flags, "#", "\\#");
+      }
       *this->FlagFileStream << language << "_FLAGS" << arch << " = " << flags
                             << "\n\n";
     }

Tests/TryCompile/CMakeLists.txt

@@ -187,7 +187,7 @@ try_compile(SHOULD_FAIL_DUE_TO_BAD_SOURCE
 if(SHOULD_FAIL_DUE_TO_BAD_SOURCE AND NOT CMAKE_GENERATOR MATCHES "Watcom WMake|NMake Makefiles")
   string(REPLACE "\n" "\n  " output "  ${output}")
   message(SEND_ERROR "try_compile with bad#source.c did not fail:\n${output}")
-elseif(NOT output MATCHES [[(bad#source\.c|bad\\)]])
+elseif(NOT output MATCHES [[(bad#source\.c|bad\.c|bad')]])
   string(REPLACE "\n" "\n  " output "  ${output}")
   message(SEND_ERROR "try_compile with bad#source.c failed without mentioning bad source:\n${output}")
 else()