fix clang++ compile warnings

[bsergean]

Hi there, small PR to fix some warnings.

[bsergean]

core/src/rtm.c:528:17: warning: ISO C++11 does not allow conversion from string literal to
      'char *' [-Wwritable-strings]
    auto_port = "80";
                ^
core/src/rtm.c:532:17: warning: ISO C++11 does not allow conversion from string literal to
      'char *' [-Wwritable-strings]
    auto_port = "443";
                ^
2 warnings generated.

rtm.h:35:9: warning: 'USE_TLS' macro redefined [-Wmacro-redefined]
#define USE_TLS
        ^
my_file.h:13:9: note: previous definition is here
#define USE_TLS 1

[satori-avasenin]

We are working to remove USE_TLS define completely because our main endpoint works only on TLS so it doesn't make any sense to build SDK without TLS. I will cherry-pick you changes about string literal to cstring convention. Will push them until the end of week.

[bsergean]

Sounds good / I reverted that change and pushed to my master, maybe it will make merging this easier.

[bsergean]

I've added something unrelated to this commit ... I should have made a branch.

f640ceb adds 2 files that are required if one wants to build with autotools.