Added support for ASMJIT_INSTALL variable

This can be used to disable installation of asmjit files together with the parent project. If you're building a package with CPack (make package), you don't necessarily want asmjit headers and library to be included in your package.

If ASMJIT_INSTALL is TRUE, everything is as usual. If it's FALSE, nothing will be installed with make install and make package (which is the default).