Add an option in CMake to build cpprestsdk using /MT

[howryu]

I am building cpprestsdk by myself on widows using cmake. Right now it worked as excepted that by default cmake is built with /MD option. It would be better to have an option to build with /MT. I know I can use vcpkg, but I am more comfortable with building the package by myself.

[ioannis-e]

Modifications to the library are needed to build with /MT successfully and prevent it from crashing as return values like std::string from exported functions in dynamic libraries will tend to corrupt the heap and crash the application at the point of class destruction when the library is built as a dynamic library with /MT.

I am currently working on a solution about both of the issues above, but changes are substantial and require extensive testing.

[ioannis-e]

@howryu you may want to check out #884 to assist with testing out the modifications to the library. You will have to make CMake modifications on your own.