cmake: Fix compiling if CFLAGS or CXXFLAGS are set

[Biswa96]

This fixes checking _mm_extract_epi64 intrinsic when CFLAGS or CXXFLAGS
are set. The -msse4.1 option is appended with existing compiler flags.
Otherwise, _mm_extract_epi64 check fails without the -msse4.1 option.
For example, the issue has been seen with the following variables in
mingw-w64 gcc 64-bit toolchain.

export CPPFLAGS="-D_FORTIFY_SOURCE=0 -D__USE_MINGW_ANSI_STDIO=1"
export CFLAGS="-mthreads -mtune=generic -O2 -pipe"
export CXXFLAGS="-mthreads -mtune=generic -O2 -pipe"
export LDFLAGS="-pipe -static-libgcc -static-libstdc++"

[adamjw24]

Could you elaborate more? Why does add_compile_options ignore your system-wide CPPFLAGS and CFLAGS settings?

[adamjw24]

Obsoleted by #268