Makefile: Workaround mingw32-make trailing backslash trouble (#15546)

When given the command line

  tool a\ b c

mingw32-make incorrectly passes "a b" and "c" to the tool.  When given
the command line

  tool a\ b "c"

mingw32-make correctly passes "a\", "b", and "c" to the tool.

Since commit v3.1.0-rc1~861^2 (MSVC: Add properties to configure
compiler PDB files, 2014-02-24) we pass the compiler pdb option to
MS-style compiler tools as "/Fd<dir>\" but mingw32-make may consume
the backslash as escaping a following space as described above.
Workaround this problem by changing the backslash to a forward
slash as had been used prior to the above commit.

Source/cmLocalUnixMakefileGenerator3.h

@@ -95,6 +95,7 @@ public:
    * Set to true if the make tool being used is MinGW Make.
    */
   void SetMinGWMake(bool v)  {this->MinGWMake = v;}
+  bool IsMinGWMake() const { return this->MinGWMake; }
 
   /**
    * Set to true if the make tool being used is NMake.

Source/cmMakefileTargetGenerator.cxx

@@ -23,6 +23,7 @@
 #include "cmComputeLinkInformation.h"
 #include "cmCustomCommandGenerator.h"
 #include "cmGeneratorExpression.h"
+#include "cmAlgorithms.h"
 
 #include "cmMakefileExecutableTargetGenerator.h"
 #include "cmMakefileLibraryTargetGenerator.h"
@@ -668,6 +669,15 @@ cmMakefileTargetGenerator
     this->Convert(targetFullPathCompilePDB,
                   cmLocalGenerator::START_OUTPUT,
                   cmLocalGenerator::SHELL);
+
+  if (this->LocalGenerator->IsMinGWMake() &&
+      cmHasLiteralSuffix(targetOutPathCompilePDB, "\\"))
+    {
+    // mingw32-make incorrectly interprets 'a\ b c' as 'a b' and 'c'
+    // (but 'a\ b "c"' as 'a\', 'b', and 'c'!).  Workaround this by
+    // avoiding a trailing backslash in the argument.
+    targetOutPathCompilePDB[targetOutPathCompilePDB.size()-1] = '/';
+    }
   }
   cmLocalGenerator::RuleVariables vars;
   vars.RuleLauncher = "RULE_LAUNCH_COMPILE";