Set CYGDRIVE_MOUNT at configure time for cygwin-wrapper performance enhancement to work.

Thanks to Brian Ryner <bryner@brianryner.com> for the patch.
Bug #206643 r=cls a=mkaply

config/autoconf.mk.in

@@ -183,6 +183,7 @@ XLDFLAGS	= @XLDFLAGS@
 XLIBS		= @XLIBS@
 
 CYGWIN_WRAPPER	= @CYGWIN_WRAPPER@
+CYGDRIVE_MOUNT	= @CYGDRIVE_MOUNT@
 AIX_SHLIB_BIN	= @AIX_SHLIB_BIN@
 AR		= @AR@
 AR_FLAGS	= @AR_FLAGS@

config/config.mk

@@ -771,3 +771,9 @@ endif # WINNT
 
 # Use nsinstall in copy mode to install files on the system
 SYSINSTALL	= $(NSINSTALL) -t
+
+ifeq ($(OS_ARCH),WINNT)
+ifneq (,$(CYGDRIVE_MOUNT))
+export CYGDRIVE_MOUNT
+endif
+endif

configure.in

@@ -5315,6 +5315,8 @@ mingw*|cygwin*|msvc*|mks*)
     if test -z "${_CYGWIN_PERL}"; then
         PERL="\$(CYGWIN_WRAPPER) $PERL"
     fi
+    CYGDRIVE_MOUNT=`mount -p | awk '{ if (/^\//) { print $1; exit } }'`
+    AC_SUBST(CYGDRIVE_MOUNT)
     ;;
 esac