Don't reset PYTHON_VERSION if it was explicitly passed in

...along with PYTHON, PYTHON_CFLAGS, and PYTHON_LIBS.  (PYTHON_VERSION is
apparently set by AM_PATH_PYTHON.  When explicitly passing in PYTHON_CFLAGS and
PYTHON_LIBS under --enable-python=system, AM_PATH_PYTHON does not get called,
but PYTHON_VERSION needs to be set, so needs to be passed in explicitly.)

Change-Id: Ia8c3877fffd8dcc179a56be04627c61a667d8cb0
Reviewed-on: https://gerrit.libreoffice.org/73662
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -8365,12 +8365,16 @@ AC_SUBST(XMLLINT)
 # ./configure PYTHON=/usr/bin/python
 # =====================================================================
 if test "$build_os" != "cygwin" -a "$enable_python" != fully-internal; then
-    # This allows a lack of system python with no error, we use internal one in that case.
-    AM_PATH_PYTHON([2.7],, [:])
-    # Clean PYTHON_VERSION checked below if cross-compiling
-    PYTHON_VERSION=""
-    if test "$PYTHON" != ":"; then
+    if test -n "$PYTHON"; then
         PYTHON_FOR_BUILD=$PYTHON
+    else
+        # This allows a lack of system python with no error, we use internal one in that case.
+        AM_PATH_PYTHON([2.7],, [:])
+        # Clean PYTHON_VERSION checked below if cross-compiling
+        PYTHON_VERSION=""
+        if test "$PYTHON" != ":"; then
+            PYTHON_FOR_BUILD=$PYTHON
+        fi
     fi
 fi
 AC_SUBST(PYTHON_FOR_BUILD)