Must use the Unix form of a pathname in a test command on WSL

Change-Id: Ib785ba0b22a555136a64c97c50aa2c17a75f6c03
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/104774
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>
diff --git a/configure.ac b/configure.ac
index 9b2f66e..47b9330 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3942,7 +3942,8 @@ if test "$_os" = "WINNT"; then
    # Find the version of devenv.exe
    # MSVC 2017 devenv does not start properly from a DOS 8.3 path
    DEVENV=$(cygpath -lm "$VC_PRODUCT_DIR/../Common7/IDE/devenv.exe")
    if test ! -e "$DEVENV"; then
    DEVENV_unix=$(cygpath -u "$DEVENV")
    if test ! -e "$DEVENV_unix"; then
        AC_MSG_WARN([No devenv.exe found - this is expected for VC++ Build Tools])
    fi