update main CMakeLists to set mpfr optional

[nkx111]

This PR clarified some optional dependencies for the framework: curl, mpfr, mySQL, and ROOT eve library. Those items will contribute to "REST Features" output when configuring cmake:

...
REST will be installed in: xxxx
===========================================================
System is: Linux
Packages: restG4 
Libraries: RestFramework;RestConnectors;RestDetector;RestGeant4;RestLegacy;RestRaw;RestTrack
Features: Eve;CURL,Garfield
===========================================================

mpfr library is made optional. A new cmake flag is introduced. By setting -DREST_MPFR=OFF one can exclude the compilation of the class TRestComplex. By default mpfr is OFF.

curl library finding is moved to main CMakeLists.