Set CPPFLAGS instead of the INCLUDE environment variable

INCLUDE is not exported from WSL to Win32 programs without including
it in the list of environment variable names in the WSLENV environment
variable, and I think we want to minimize the number that need to be
mentioned there.

Should not affect Cygwin builds.

Change-Id: I0af480873badac446305e396b69eec360eb8a270
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/105098
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>

configure.ac

@@ -3967,7 +3967,8 @@ if test "$_os" = "WINNT"; then
         # Remove /cl.exe from CC case insensitive
         AC_MSG_NOTICE([found Visual C++ $vcyear])
 
-        export INCLUDE=`cygpath -d "$COMPATH/Include"`
+        main_include_dir=`cygpath -d -m "$COMPATH/Include"`
+        CPPFLAGS="$CPPFLAGS -I$main_include_dir"
 
         PathFormat "$COMPATH"
         COMPATH=`win_short_path_for_make "$formatted_path"`