Fix unused functions on Android

[basilgello]

• Implement USE_OS_TZDB for Android
• Ensure Android private functions dont show up
• Fix unused functions on Android

[basilgello]

@HowardHinnant Now no more unused functions in Android! Apart from that, tests against NDK r27b caught the following static assertion failures:

/tmp/date/test/date_test/year.pass.cpp:116:19: error: static assertion failed due to requirement 'std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int, std::ratio<86400, 1>>>(year::min() / jan / 1) - std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int, std::ratio<86400, 1>>>(1970_y / jan / 1) >= as<long long>(duration<long, std::ratio<60, 1>>::min())':
  116 |     static_assert(sys_days(year::min()/jan/1)  - sys_days(1970_y/jan/1)
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  117 |         >= as<int64_t>(minutes::min()), "");
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[ 75%] Linking CXX executable date_test_pass_op_div_year_month_bin
[ 75%] Linking CXX executable clock_cast_test_pass_constexpr_bin
[ 75%] Built target date_test_pass_last_bin
/tmp/date/test/date_test/year.pass.cpp:130:19: error: static assertion failed due to requirement 'std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int, std::ratio<86400, 1>>>(year::max() / dec / 31) - std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<int, std::ratio<86400, 1>>>(1970_y / jan / 1) <= as<long long>(duration<long, std::ratio<60, 1>>::max())':
  130 |     static_assert(sys_days(year::max()/dec/31) - sys_days(1970_y/jan/1)
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  131 |         <= as<int64_t>(minutes::max()), "");
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Kodi compiles and passes integration tests just fine, buut you might still find the issue interesting!

[basilgello]

Reproducer:

#!/bin/sh

set -e

cd date
rm -rf build
mkdir build
cd build
cmake \
        -DCMAKE_TOOLCHAIN_FILE=/tmp/android-ndk-r27b/build/cmake/android.toolchain.cmake \
        -DANDROID_ABI=armeabi-v7a \
        -DANDROID_PLATFORM=android-21 \
        -DBUILD_TZ_LIB=ON \
        -DUSE_SYSTEM_TZ_DB=ON \
        -DENABLE_DATE_TESTING=ON \
        -DCMAKE_CXX_FLAGS=-Wall \
        -DCMAKE_CXX_FLAGS=-Wall \
        ..
make VERBOSE=1
cmake --build . --target testit -j
make test VERBOSE=1

[basilgello]

You download https://dl.google.com/android/repository/android-ndk-r27b-darwin.dmg and unpack stuff so /tmp/android-ndk-r27b is NDK root directory, just in case you wanna try it out

[HowardHinnant]

date/src/tz.cpp:656:1: error: unknown type name 'Wstatic'; did you mean 'static'?
  656 | Wstatic
      | ^~~~~~~
      | static
1 error generated.

[basilgello]

Typo-ed :( Fixed!

[HowardHinnant]

Merged (with a couple of trivial fixes): 447f5a3