Changes to accomodate build on Windows

[davidtgillard]

• removed librt dependency
• specify C++11 as standard
• remove some compiler flags
• added WindowsUtcTimeSource singleton class (determines if GetSystemTimePreciseAsFileTime is available)
• updated DateTime::Now to use WindowsUtcTimeSource if compiling on Windows

[dnwrnr]

Add back compiler flags for non windows.
Why remove LIBRARY from install(TARGETS sgp4s LIBRARY DESTINATION lib)
Given this is c++11 compliant, you should be using std::chrono::system_clock etc

[davidtgillard]

Apologies for the delay in response.

• Compiler flags: no problem.
• I removed the LIBRARY target specifier was removed from the install command for sgp4s because shared libraries are treated as runtime objects. It exhibits the same behaviour without the target specifier when I build on linux/mac.
• From what I've read, the epoch of std::chrono::system_clock is de facto the unix epoch (seems to be the case between the major compilers)...but I suppose it's not technically guaranteed. The alternative is that ugly workaround - I'll do whichever you prefer.