Fixes #40: add _DEFAULT_SOURCE back for Linux build

This commit fixes the improper patch in the pull request #7.

When building pforth on the Linux system with glibc version before
2.19, defining _GNU_SOURCE is needed for using ECHOCTL in the
source. However, the later version of glibc uses _DEFAULT_SOURCE
instead of _GNU_SOURCE for the same purpose, we will need both symbols
for both older glibc and newer glibc.