diff -urN a/build/autoconf/config.status.m4 b/build/autoconf/config.status.m4
--- a/build/autoconf/config.status.m4	2015-07-13 22:34:59.729176500 +0100
+++ b/build/autoconf/config.status.m4	2015-07-13 22:47:20.516343000 +0100
@@ -87,7 +87,6 @@
 case "$host_os" in
 mingw*)
     WIN_TOP_SRC=`cd $srcdir; pwd -W`
-    encoding=mbcs
     ;;
 esac
 AC_SUBST(WIN_TOP_SRC)
diff -urN a/build/autoconf/python-virtualenv.m4 b/build/autoconf/python-virtualenv.m4
--- a/build/autoconf/python-virtualenv.m4	2015-07-13 22:34:59.729176500 +0100
+++ b/build/autoconf/python-virtualenv.m4	2015-07-13 22:47:20.520343200 +0100
@@ -56,14 +56,7 @@
     $_virtualenv_topsrcdir $MOZ_BUILD_ROOT $MOZ_BUILD_ROOT/_virtualenv \
     $_virtualenv_topsrcdir/build/virtualenv_packages.txt || exit 1
 
-  case "$host_os" in
-  mingw*)
-    PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe
-    ;;
-  *)
-    PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
-    ;;
-  esac
+  PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
 fi
 
 AC_SUBST(PYTHON)
diff -urN a/configure b/configure
--- a/configure	2015-07-13 22:37:44.550806000 +0100
+++ b/configure	2015-07-13 22:47:20.534344000 +0100
@@ -1367,7 +1367,7 @@
   exit 1
   break
 fi
-MOZ_BUILD_ROOT=`pwd -W 2>/dev/null || pwd`
+MOZ_BUILD_ROOT=`pwd`
 
 
 
@@ -1474,14 +1474,7 @@
     $_virtualenv_topsrcdir $MOZ_BUILD_ROOT $MOZ_BUILD_ROOT/_virtualenv \
     $_virtualenv_topsrcdir/build/virtualenv_packages.txt || exit 1
 
-  case "$host_os" in
-  mingw*)
-    PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe
-    ;;
-  *)
-    PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
-    ;;
-  esac
+  PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
 fi
 
 
diff -urN a/configure.in b/configure.in
--- a/configure.in	2015-07-13 22:36:56.161466600 +0100
+++ b/configure.in	2015-07-13 22:47:20.772353700 +0100
@@ -133,7 +133,7 @@
   exit 1
   break
 fi
-MOZ_BUILD_ROOT=`pwd -W 2>/dev/null || pwd`
+MOZ_BUILD_ROOT=`pwd`
 
 MOZ_PYTHON
 
diff -urN a/js/src/configure b/js/src/configure
--- a/js/src/configure	2015-07-13 22:38:51.104908900 +0100
+++ b/js/src/configure	2015-07-13 22:47:20.782353700 +0100
@@ -1061,7 +1061,7 @@
   exit 1
   break
 fi
-MOZ_BUILD_ROOT=`pwd -W 2>/dev/null || pwd`
+MOZ_BUILD_ROOT=`pwd`
 
 
 # Check whether --with-dist-dir or --without-dist-dir was given.
@@ -4404,14 +4404,7 @@
     $_virtualenv_topsrcdir $MOZ_BUILD_ROOT $MOZ_BUILD_ROOT/_virtualenv \
     $_virtualenv_topsrcdir/build/virtualenv_packages.txt || exit 1
 
-  case "$host_os" in
-  mingw*)
-    PYTHON=`cd $MOZ_BUILD_ROOT && pwd -W`/_virtualenv/Scripts/python.exe
-    ;;
-  *)
-    PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
-    ;;
-  esac
+  PYTHON=$MOZ_BUILD_ROOT/_virtualenv/bin/python
 fi
 
 
diff -urN a/python/virtualenv/virtualenv.py b/python/virtualenv/virtualenv.py
--- a/python/virtualenv/virtualenv.py	2015-07-13 22:35:33.117337000 +0100
+++ b/python/virtualenv/virtualenv.py	2015-07-13 22:47:20.797353800 +0100
@@ -45,6 +45,7 @@
 is_pypy = hasattr(sys, 'pypy_version_info')
 is_win = (sys.platform == 'win32')
 is_cygwin = (sys.platform == 'cygwin')
+is_msys = (sys.platform == 'msys')
 is_darwin = (sys.platform == 'darwin')
 abiflags = getattr(sys, 'abiflags', '')
 
@@ -1276,7 +1277,7 @@
         executable = sys.executable
         shutil.copyfile(executable, py_executable)
         make_exe(py_executable)
-        if is_win or is_cygwin:
+        if is_win or is_cygwin or is_msys:
             pythonw = os.path.join(os.path.dirname(sys.executable), 'pythonw.exe')
             if os.path.exists(pythonw):
                 logger.info('Also created pythonw.exe')
@@ -1290,11 +1291,19 @@
                 logger.info('Removed python_d.exe as it is no longer at the source')
                 os.unlink(python_d_dest)
             # we need to copy the DLL to enforce that windows will load the correct one.
-            # may not exist if we are cygwin.
-            py_executable_dll = 'python%s%s.dll' % (
-                sys.version_info[0], sys.version_info[1])
-            py_executable_dll_d = 'python%s%s_d.dll' % (
-                sys.version_info[0], sys.version_info[1])
+            try:
+                # Cygwin, msys2 and MSYS2's mingw-w64-{i686,x86-64}-python all support this.
+                # .. disabled for now on msys2 as there's a dll address overlap issue:
+                # 0 [main] python 824 child_info_fork::abort: address space needed by 'msys-python2.7.dll' (0x6F0000) is already occupied
+                import sysconfig
+                py_executable_dll = 'unused' if is_msys else sysconfig.get_config_var('DLLLIBRARY')
+                py_executable_dll_d = 'unused'
+            except:
+                # .. but CPython doesn't.
+                py_executable_dll = 'python%s%s.dll' % (
+                    sys.version_info[0], sys.version_info[1])
+                py_executable_dll_d = 'python%s%s_d.dll' % (
+                    sys.version_info[0], sys.version_info[1])
             pythondll = os.path.join(os.path.dirname(sys.executable), py_executable_dll)
             pythondll_d = os.path.join(os.path.dirname(sys.executable), py_executable_dll_d)
             pythondll_d_dest = os.path.join(os.path.dirname(py_executable), py_executable_dll_d)