Fix for universal binaries on macOS

Fix the issue of a
    clang: error: no such file or directory: 'x86_64'
error on building universal binaries on macOS.

It is also necessary to specify:
    -DCMAKE_C_FLAGS="-DPNG_ARM_NEON_OPT=0"

Example CMake command:
    cmake -S libpng \
          -B build_libpng \
          -DCMAKE_OSX_ARCHITECTURES="arm64;x86_64" \
          -DCMAKE_C_FLAGS="-DPNG_ARM_NEON_OPT=0"

Signed-off-by: Cosmin Truta <ctruta@gmail.com>
2 files changed