configure: Miscellaneous minor changes to config file handling

- Move generating config.fate to a more sensible place.
- Move printing warnings to a more appropriate place.
- Improve "generated by" comment in libavutil/avconfig.h.
- Drop pointless informative output about generating config files.
- Write a standard comment header to config.asm as well.

configure

@@ -4109,12 +4109,6 @@ esac
 check_compile_assert flt_lim "float.h limits.h" "DBL_MAX == (double)DBL_MAX" ||
     add_cppflags '-I\$(SRC_PATH)/compat/float'
 
-esc(){
-    echo "$*" | sed 's/%/%25/g;s/:/%3a/g'
-}
-
-echo "config:$arch:$subarch:$cpu:$target_os:$(esc $cc_ident):$(esc $LIBAV_CONFIGURATION)" > avbuild/config.fate
-
 check_cpp_condition stdlib.h "defined(__PIC__) || defined(__pic__) || defined(PIC)" && enable_weak pic
 
 set_default $PATHS_LIST
@@ -5192,12 +5186,18 @@ fi
 
 echo "License: $license"
 
-echo "Creating configuration files ..."
-
 fi # test "$quiet" != "yes"
 
+test -n "$WARNINGS" && printf "\n$WARNINGS"
+
 test -e Makefile || echo "include $source_path/Makefile" > Makefile
 
+esc(){
+    echo "$*" | sed 's/%/%25/g;s/:/%3a/g'
+}
+
+echo "config:$arch:$subarch:$cpu:$target_os:$(esc $cc_ident):$(esc $LIBAV_CONFIGURATION)" > avbuild/config.fate
+
 config_files="$TMPH avbuild/config.mak"
 
 cat > avbuild/config.mak <<EOF
@@ -5335,7 +5335,9 @@ test -n "$malloc_prefix" &&
 
 if enabled x86asm; then
     append config_files $TMPASM
-    printf '' >$TMPASM
+    cat > $TMPASM <<EOF
+; Automatically generated by configure - do not modify!
+EOF
 fi
 
 enabled getenv || echo "#define getenv(x) NULL" >> $TMPH
@@ -5355,7 +5357,7 @@ touch avbuild/.config
 enabled x86asm && cp_if_changed $TMPASM config.asm
 
 cat > $TMPH <<EOF
-/* Generated by ffconf */
+/* Generated by libav configure */
 #ifndef AVUTIL_AVCONFIG_H
 #define AVUTIL_AVCONFIG_H
 EOF
@@ -5383,8 +5385,6 @@ print_enabled_components(){
 print_enabled_components libavcodec/bsf_list.c AVBitStreamFilter bitstream_filters $BSF_LIST
 print_enabled_components libavformat/protocol_list.c URLProtocol url_protocols $PROTOCOL_LIST
 
-test -n "$WARNINGS" && printf "\n$WARNINGS"
-
 # Settings for pkg-config files
 
 cat > avbuild/config.sh <<EOF