And keep ndk-patches in 23c/ subdirectory. Run
termux_step_setup_toolchain_23c only if TERMUX_NDK_VERSION equals 23c.
This is a step towards having the possibility to use different NDK
versions. Using a different NDK version than the one termux
officially supports should *really* not be done except for
testing/debug/development reasons, or if it is strictly necessary to
be able to compile a program (for example for packages that need a
fortran compiler, which at the moment is only supported with old
gcc-using NDKs).
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