Revert "Use mruby default CFLAGS"

prefer Debian Default CFLAGS than mruby Default.

This reverts commit a9464feb5fb394e372c75f82ebc1b3763fc8e864.
STRTOUL macro is not support non-C99 and non-C11 mode.
Therefore, if this was compiled without "-std=gnu99" option, we get
the following error.

----
error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
----

This fixes syntax of STRTOUL macro. And this changes so that it can be compiled
when C99 or C11 mode has not been specified.

Signed-off-by: Akira Mitsui <murase.syuka@gmail.com>