configure: Use pkg-config for fdk-aac

Makes the life of the people already setting PKG_CONFIG_PATH simpler.

configure

@@ -4288,7 +4288,7 @@ enabled gnutls            && require_pkg_config gnutls gnutls/gnutls.h gnutls_gl
 enabled libbs2b           && require_pkg_config libbs2b bs2b.h bs2b_open
 enabled libdcadec         && require libdcadec libdcadec/dca_context.h dcadec_context_create -ldcadec
 enabled libfaac           && require2 libfaac "stdint.h faac.h" faacEncGetVersion -lfaac
-enabled libfdk_aac        && require libfdk_aac fdk-aac/aacenc_lib.h aacEncOpen -lfdk-aac
+enabled libfdk_aac        && require_pkg_config fdk-aac "fdk-aac/aacenc_lib.h" aacEncOpen
 enabled libfontconfig     && require_pkg_config fontconfig "fontconfig/fontconfig.h" FcInit
 enabled libfreetype       && require_pkg_config freetype2 "ft2build.h FT_FREETYPE_H" FT_Init_FreeType
 enabled libgsm            && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do