configure: really print an error if the compiler cannot be found

configure.ac

@@ -71,7 +71,8 @@ AC_ARG_VAR([DESKTOP_FILE_VALIDATE], [Validator for desktop entry files])
 AC_CHECK_PROGS(DESKTOP_FILE_VALIDATE, [${DESKTOP_FILE_VALIDATE} desktop-file-validate], :)
 AC_PATH_PROG(YASM, yasm)
 AC_ARG_VAR([BUILDCC], [Build system C11 or C99 compiler command])
-AC_PATH_PROGS(BUILDCC, [c11-gcc c11 c99-gcc c99], [
+AC_PATH_PROGS(BUILDCC, [c11-gcc c11 c99-gcc c99], [false])
+AS_IF([test "$BUILDCC" = "false"], [
   AC_MSG_ERROR([Cannot find native C99 compiler: please define BUILDCC.])
 ])