Decide between c89 or c99 and specify it in CMake

[piedar]

c99 is preferable because it includes a bool type and it allows variable declarations in for loops. Only reason I can think to stay with c89 is to support old or exotic compilers. If you are using a compiler without c99 support, please comment here.

Either way, we should specify the target standard in CMakeLists.txt (possibly with -pedantic) so that the choice is explicit and consistent.

[smokhov]

@piedar -- in #538 you asked whether my compiler supports C99, the answer is yes, GCC 4.4.7 that comes with EL6 distros, has options like -std=gnu99, but cmake did not generate them into Makefile(s), so make failed.