configure: use AC_CHECK_PROG(S) instead of AC_PATH_PROG(S)

Where the search paths list is not changed, using AC_PATH_PROG(S) has
no useful effects and makes manual override more difficult.

(Compare to Qt code using different search paths.)

configure.ac

@@ -70,7 +70,7 @@ AM_PROG_AS
 AC_ARG_VAR([DESKTOP_FILE_VALIDATE], [Validator for desktop entry files])
 AC_CHECK_PROGS(DESKTOP_FILE_VALIDATE, [${DESKTOP_FILE_VALIDATE} desktop-file-validate], :)
 AC_ARG_VAR([BUILDCC], [Build system C11 or C99 compiler command])
-AC_PATH_PROGS(BUILDCC, [c11-gcc c11 c99-gcc c99], [false])
+AC_CHECK_PROGS(BUILDCC, [c11-gcc c11 c99-gcc c99], [false])
 AS_IF([test "$BUILDCC" = "false"], [
   AC_MSG_ERROR([Cannot find native C99 compiler: please define BUILDCC.])
 ])
@@ -1513,7 +1513,7 @@ then
       AC_MSG_ERROR([Could not find lua. Lua is needed for some interfaces (rc, telnet, http) as well as many other custom scripts. Use --disable-lua to ignore this error.])
   fi
   AC_ARG_VAR([LUAC], [LUA byte compiler])
-  AC_PATH_PROGS(LUAC,[${LUAC} luac], [false])
+  AC_CHECK_PROGS(LUAC, [${LUAC} luac], [false])
   AS_IF([test "${LUAC}" = "false"], [
     AC_MSG_ERROR([Could not find the LUA byte compiler.])
   ])
@@ -3619,7 +3619,7 @@ dnl  Chromecast streaming support
 dnl
 m4_pushdef([protobuf_lite_version], 2.5.0)
 AC_ARG_VAR(PROTOC, [protobuf compiler])
-AC_PATH_PROGS(PROTOC, protoc, no)
+AC_CHECK_PROGS(PROTOC, protoc, no)
 PKG_WITH_MODULES([CHROMECAST],[protobuf-lite >= protobuf_lite_version], [
     AS_IF([test "x${PROTOC}" != "xno"], [
         build_chromecast="yes"