Added Interface with CRYPTOPP_INSTALL_PREFIX definition and Added test for compilation with prefix

[Vollstrecker]

k, I now added the Interface and the code in the test to use it. This could also be the example you need for #9 (thanks to aMule). If I see this correct, the find-package test is run after all install tests, so it picks up the one with the prefix.

@abdes Have you an idea to reorder the tests so first all without and then all with prefix are run? Or do you think one test that uses the defined prefix from the interface is enough? Imho the code doesn't care about what is defined as long as a header is found, so testing this twice could be just dropped.

[abdes]

@abdes Have you an idea to reorder the tests so first all without and then all with prefix are run? Or do you think one test that uses the defined prefix from the interface is enough? Imho the code doesn't care about what is defined as long as a header is found, so testing this twice could be just dropped.

As you said, the code does not really care, so there is no point in reordering. As long as the unit tests using prefix pass, it's ok.