make attribute CFLAGS and LDFLAGS optional and add default empty value

Actually CFLAGS and LDFLAGS are not required and can be empty.
Without default empty value if target platform in project settings was
set to "Linux" CFLAGS and LDFLAGS was initialized with NoneType.
The result was broken build and it wasn't to save/load project with
such settings.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 956, in _Build
if not builder.build() :
File "/home/developer/WorkData/PLC/beremiz/beremiz/targets/toolchain_gcc.py", line 96, in build
Builder_CFLAGS = ' '.join(self.getBuilderCFLAGS())
TypeError: sequence item 0: expected string, NoneType found

     1

# SOME DESCRIPTIVE TITLE.

     2

# Copyright (C) is 21st century inquisition

     3

# This file is distributed under the same license as the Beremiz package.

     4

# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.

     5

#

     6

#, fuzzy

     7

msgid ""

     8

msgstr ""

     9

"Project-Id-Version: Beremiz 1.1\n"

    10

"Report-Msgid-Bugs-To: \n"

    11

"POT-Creation-Date: 2012-09-08 00:41\n"

    12

"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"

    13

"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"

    14

"Language-Team: LANGUAGE <LL@li.org>\n"

    15

"Language: \n"

    16

"MIME-Version: 1.0\n"

    17

"Content-Type: text/plain; charset=UTF-8\n"

    18

"Content-Transfer-Encoding: 8bit\n"

    19

    20

# 085a2a1c2a1043d7b09d75d0c657d2a3

    21

#: ../../manual/debug.rst:2

    22

msgid "Trace POUs instances variable"

    23

msgstr ""