Add new step to clean $PATH var by removing /mnt specific Window's %PATH% paths that cause issues with the make system

doc/build-windows.md

@@ -72,6 +72,7 @@ To build executables for Windows 64-bit, install the following dependencies:
 
 Then build using:
 
+    PATH=$(echo "$PATH" | sed -e 's/:\/mnt.*//g') # strip out problematic Windows %PATH% imported var
     cd depends
     make HOST=x86_64-w64-mingw32
     cd ..
@@ -87,6 +88,7 @@ To build executables for Windows 32-bit, install the following dependencies:
 
 Then build using:
 
+    PATH=$(echo "$PATH" | sed -e 's/:\/mnt.*//g') # strip out problematic Windows %PATH% imported var
     cd depends
     make HOST=i686-w64-mingw32
     cd ..