swr: Refactor checks for compiler feature flags

Encapsulate the test for which flags are needed to get a compiler to
support certain features.  Along with this, give various options to try
for AVX and AVX2 support.  Ideally we want to use specific instruction
set feature flags, like -mavx2 for instance instead of -march=haswell,
but the flags required for certain compilers are different.  This
allows, for AVX2 for instance, GCC to use -mavx2 -mfma -mbmi2 -mf16c
while the Intel compiler which doesn't support those flags can fall
back to using -march=core-avx2.

This addresses a bug where the Intel compiler will silently ignore the
AVX2 instruction feature flags and then potentially fail to build.

v2: Pass preprocessor-check argument as true-state instead of
    false-state for clarity.
v3: Reduce AVX2 define test to just __AVX2__.  Additional defines suchas
    __FMA__, __BMI2__, and __F16C__ appear to be inconsistently defined
    w.r.t thier availability.
v4: Fix C++11 flags being added globally and add more logic to
    swr_require_cxx_feature_flags

Cc: <[email protected]>
Reviewed-by: Tim Rowley <[email protected]>
Tested-by: Tim Rowley <[email protected]>
Signed-off-by: Chuck Atkins <[email protected]>

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