Cross compile Crypto++ with GNUmakefile-cross

[frankmorgner]

Crypto++ recently introduced recipe for cross compiling https://www.cryptopp.com/wiki/ARM_Embedded_%28Command_Line%29. This worked for me out of the box when setting CC/CXX and could be used when compiling for android with
https://github.com/BeID-lab/eIDClientCore/blob/master/eIDClientCore/android/Makefile