Save #define flags into libsnark-config.h

[tromer]

Currently we have a bunch of #define flags fed by the makefile as g++ -D... arguments during the build. But after we build a library file (libsnark.a or libsnark.so), the information on choice of flags is lost. Later, if a user compiling against the libsnark library happens to use different flags, all bets are off. This is especially bad because some of our code is in .c files (so affeceted by the flags when compiling libsnark), and other is in .tcc files (so affected by the flags when compiling the user's app).

Let's save the flags into a libsnark_config.h file and #include it from all pertinent places in libsnark, instead of passing g++ -D arguments. Then, make install can then install it with the rest of the headers.

[tromer]

Related to #65.