configure: add optional pkg-config helper and use it.

The require variant dies if the package is not present.
The check variant does not import the flags to the used list.
The new variant imports the flags if the package is present
but does not die if it is not.
The new call graph is: require -> use -> check.

Use use_pkg_config for libx264 and libsmbclient: more readable
and three external call less per library.

configure

@@ -1203,13 +1203,17 @@ require_cpp(){
     check_lib_cpp "$headers" "$classes" "$@" || die "ERROR: $name not found"
 }
 
-require_pkg_config(){
+use_pkg_config(){
     pkg="$1"
-    check_pkg_config "$@" || die "ERROR: $pkg not found"
+    check_pkg_config "$@" || return 1
     add_cflags    $(get_safe ${pkg}_cflags)
     add_extralibs $(get_safe ${pkg}_libs)
 }
 
+require_pkg_config(){
+    use_pkg_config "$@" || die "ERROR: $pkg not found"
+}
+
 require_libfreetype(){
     log require_libfreetype "$@"
     pkg="freetype2"
@@ -4875,9 +4879,8 @@ enabled libquvi           && require_pkg_config libquvi quvi/quvi.h quvi_init
 enabled librtmp           && require_pkg_config librtmp librtmp/rtmp.h RTMP_Socket
 enabled libschroedinger   && require_pkg_config schroedinger-1.0 schroedinger/schro.h schro_init
 enabled libshine          && require_pkg_config shine shine/layer3.h shine_encode_buffer
-enabled libsmbclient      && { { check_pkg_config smbclient libsmbclient.h smbc_init &&
-                                 require_pkg_config smbclient libsmbclient.h smbc_init; } ||
-                                 require smbclient libsmbclient.h smbc_init -lsmbclient; }
+enabled libsmbclient      && { check_pkg_config smbclient libsmbclient.h smbc_init ||
+                               require smbclient libsmbclient.h smbc_init -lsmbclient; }
 enabled libsoxr           && require libsoxr soxr.h soxr_create -lsoxr
 enabled libssh            && require_pkg_config libssh libssh/sftp.h sftp_init
 enabled libspeex          && require_pkg_config speex speex/speex.h speex_decoder_init -lspeex
@@ -4903,8 +4906,7 @@ enabled libvpx            && {
     enabled libvpx_vp9_encoder && { check_lib2 "vpx/vpx_encoder.h vpx/vp8cx.h" "vpx_codec_vp9_cx VP9E_SET_AQ_MODE" -lvpx || disable libvpx_vp9_encoder; } }
 enabled libwavpack        && require libwavpack wavpack/wavpack.h WavpackOpenFileOutput  -lwavpack
 enabled libwebp           && require_pkg_config "libwebp >= 0.2.0" webp/encode.h WebPGetEncoderVersion
-enabled libx264           && { { check_pkg_config x264 "stdint.h x264.h" x264_encoder_encode &&
-                                 require_pkg_config x264 "stdint.h x264.h" x264_encoder_encode; } ||
+enabled libx264           && { use_pkg_config x264 "stdint.h x264.h" x264_encoder_encode ||
                                { require libx264 x264.h x264_encoder_encode -lx264 &&
                                  warn "using libx264 without pkg-config"; } } &&
                              { check_cpp_condition x264.h "X264_BUILD >= 118" ||