Warnings when building with clang

[Chuck Walbourn]

The library will build with clang, but generates a number of warnings.

Note that DirectXMesh assumes you are using C++14, so you should use -std=c++14 with clang along with the newer releases of DirectXMath (3.12 or later)

[Chuck Walbourn]

Beyond some conformance fixes, many of the warnings are noise that need to be suppressed to get a 'clean' build:

#ifdef __clang__
#pragma clang diagnostic ignored "-Wc++98-compat"
#pragma clang diagnostic ignored "-Wc++98-compat-pedantic"
#pragma clang diagnostic ignored "-Wc++98-compat-local-type-template-args"
#pragma clang diagnostic ignored "-Wcovered-switch-default"
#pragma clang diagnostic ignored "-Wdouble-promotion"
#pragma clang diagnostic ignored "-Wfloat-equal"
#pragma clang diagnostic ignored "-Wglobal-constructors"
#pragma clang diagnostic ignored "-Wgnu-anonymous-struct"
#pragma clang diagnostic ignored "-Wimplicit-fallthrough"
#pragma clang diagnostic ignored "-Wlanguage-extension-token"
#pragma clang diagnostic ignored "-Wmissing-prototypes"
#pragma clang diagnostic ignored "-Wmissing-variable-declarations"
#pragma clang diagnostic ignored "-Wnested-anon-types"
#pragma clang diagnostic ignored "-Wnonportable-include-path"
#pragma clang diagnostic ignored "-Wnonportable-system-include-path"
#pragma clang diagnostic ignored "-Wreserved-id-macro"
#pragma clang diagnostic ignored "-Wshadow-field-in-constructor"
#pragma clang diagnostic ignored "-Wsign-conversion"
#pragma clang diagnostic ignored "-Wswitch-enum"
#pragma clang diagnostic ignored "-Wtautological-type-limit-compare"
#pragma clang diagnostic ignored "-Wundefined-reinterpret-cast"
#pragma clang diagnostic ignored "-Wunused-const-variable"
#pragma clang diagnostic ignored "-Wunknown-pragmas"
#endif

[Chuck Walbourn]

Fixed the conformance-related warnings in this commit