Generate a pkg-config file
test: do not use SIMDPP_ARCH_NAMESPACE_END

The files are never read by Doxygen, thus we don't need to wrap uses of
SIMDPP_ARCH_NAMESPACE by DOXYGEN_SHOULD_SKIP_THIS.
inline SIMDPP_ARCH_NAMESPACE_(BEGIN|END) macros

Doxygen needs to access simd.h to see the definition of these macros. If
this is fixed, the resulting code is not simpler. Thus we just get rid
of the macros.
Fix doxygen support, improve documentation
cache: Add generic prefetch functions
Add QtCreator project files
Add .gitignore
README: add a link to online documentation