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.