build: find qt's renamed helper libs from 5.7

build-aux/m4/bitcoin_qt.m4

@@ -473,8 +473,8 @@ AC_DEFUN([_BITCOIN_QT_FIND_LIBS_WITHOUT_PKGCONFIG],[
   ])
 
   BITCOIN_QT_CHECK(AC_CHECK_LIB([z] ,[main],,AC_MSG_WARN([zlib not found. Assuming qt has it built-in])))
-  BITCOIN_QT_CHECK(AC_CHECK_LIB([png] ,[main],,AC_MSG_WARN([libpng not found. Assuming qt has it built-in])))
+  BITCOIN_QT_CHECK(AC_SEARCH_LIBS([png_error] ,[qtpng png],,AC_MSG_WARN([libpng not found. Assuming qt has it built-in])))
-  BITCOIN_QT_CHECK(AC_CHECK_LIB([jpeg] ,[main],,AC_MSG_WARN([libjpeg not found. Assuming qt has it built-in])))
+  BITCOIN_QT_CHECK(AC_SEARCH_LIBS([jpeg_create_decompress] ,[qtjpeg jpeg],,AC_MSG_WARN([libjpeg not found. Assuming qt has it built-in])))
   BITCOIN_QT_CHECK(AC_SEARCH_LIBS([pcre16_exec], [qtpcre pcre16],,AC_MSG_WARN([libpcre16 not found. Assuming qt has it built-in])))
   BITCOIN_QT_CHECK(AC_SEARCH_LIBS([hb_ot_tags_from_script] ,[qtharfbuzzng harfbuzz],,AC_MSG_WARN([libharfbuzz not found. Assuming qt has it built-in or support is disabled])))
   BITCOIN_QT_CHECK(AC_CHECK_LIB([${QT_LIB_PREFIX}Core]   ,[main],,BITCOIN_QT_FAIL(lib$QT_LIB_PREFIXCore not found)))