compare: Fix the implementation of int64x2 comparisons on NEON

64-bit integer comparison is not available on NEON, we must implement
it in several other instructions.
Prefer to use () initializers instead of {}
Specify intermediate type in casts explicitly
convert: Add VFP implementation of to_int32x4
convert: Fix VFP implementation of to_float32x4

The current implementation was buggy as Temp was not properly aligned
and  theoretically we could have problems with badly aligned accesses.
Proper alignment requires an union, in which case mem_block becomes a
better solution.
Hide Clang-specific pragmas from other compilers
test: Enable pthread when compiling dispatcher tests

C++11 threading facility support requires -pthread both during linking
and compilation.
dispatch: Workaround a GCC bug that causes std::call_once to throw

For some reason GCC stdlib requires a strong reference to at least one
function from libpthread to force it to be linked in. Otherwise, GCC
stubs are used, which throw std::system error if one actually tries to
use them.
Fix QtCreator autocompletion by including proper __cplusplus definition
shuffle: Drop inline assembly for vswp

GCC has been fixed in some point-release after 4.7, so this is not necessary
anymore.
Use template specializations for NEON shift and shuffle optimizations

Clang frontend doesn't like NEON shift or shuffle intrinsics being
passed out of range values even if these intrinsics are never called.
Thus we need to add a separate specialization for any corner cases.
The specializations are implementation detail, as this would
complicate forward declarations.
Add a macro to disable dispatcher functionality

This lets the user to remove the dependency on boost on cxx98 branch.