No auto-usage of [s]ccache on Windows for PCH builds

Since that's a pessimisation. Any manual override
(--enable-ccache, or --disable-pch) will again make configure
auto-enable [s]ccache if binaries are found.

Change-Id: I4c0773300c792ebb1807d092d3622d90dfac7d5f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/125073
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>

configure.ac

@@ -5857,6 +5857,10 @@ if test -z "$enable_pch"; then
     if test "$_os" = "WINNT"; then
         # Enabled by default on Windows.
         enable_pch=yes
+        # never use [s]ccache on auto-enabled PCH builds, except if requested explicitely
+        if test -z "$enable_ccache"; then
+            CCACHE=""
+        fi
     else
         enable_pch=no
     fi