Fix compiler warnings for tst_cpp

[vrahane]

• The defines(_BSD_SOURCE, _DEFAULT_SOURCE) were getting defined in multiple source files
  generating warnings
• Solution is to conditionally define the above
• These were seen when making changes for Tinycbor mynewt upstream #83

9.2/mkspecs/macx-clang -F/usr/local/Cellar/qt/5.9.2/lib -o tst_cpp.o tst_cpp.cpp
In file included from tst_cpp.cpp:27:
./../../src/cborencoder.c:25:9: warning: '_BSD_SOURCE' macro redefined [-Wmacro-redefined]
#define _BSD_SOURCE 1
        ^
../../src/compilersupport_p.h:35:11: note: previous definition is here
#  define _BSD_SOURCE
          ^
In file included from tst_cpp.cpp:27:
./../../src/cborencoder.c:26:9: warning: '_DEFAULT_SOURCE' macro redefined [-Wmacro-redefined]
#define _DEFAULT_SOURCE 1
        ^
../../src/compilersupport_p.h:38:11: note: previous definition is here
#  define _DEFAULT_SOURCE
          ^
2 warnings generated.