New comment by mvf on void-packages repository

https://github.com/void-linux/void-packages/pull/23498#issuecomment-656507808

Comment:
**TL;DR**
- the TR1 flavor of `alembic` is useless
- `libatomic` is required on `armv6l*`

**Prose**

> what's up with that tr1 change

TR1 is an obsolete non-standard C++ extension that has been effectively superseded by C++11. `alembic`'s TR1 build option is merely a fallback for people stuck on GCC < 4.8. Enabling it changes `alembic`'s interfaces, making it useless on modern systems. That's also why a build option makes no sense. From the README:
```
Some examples of OPTIONS you may want or need to use include:
[...]
-DALEMBIC_LIB_USES_TR1=ON or -DALEMBIC_LIB_USES_BOOST=ON if you do not have
a C++11 capable compiler specify one of these to use TR1, or boost as
a dependency of the Alembic library.
```
My guess is that the original `alembic` packager wasn't a C++ person and thought enabling TR1 was a neat way to get it to link without patching. From `lib/Alembic/CMakeLists.txt`:
```cmake
# link in atomic if we are using tr1 and the compiler is new enough
# to want  __atomic_compare_exchange_n
IF ( ${ALEMBIC_LIB_USES_TR1} AND CMAKE_COMPILER_IS_GNUCXX AND
    CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.8 )
    TARGET_LINK_LIBRARIES( Alembic atomic )
ENDIF()
```
> without tr1, atomic shouldn't be necessary at all

`armv6l*` disagrees:
```
/usr/lib/gcc/arm-linux-gnueabihf/9.3.0/../../../../arm-linux-gnueabihf/bin/ld: ../../libAlembic.so.1.7.12: undefined reference to `__atomic_fetch_or_8'
/usr/lib/gcc/arm-linux-gnueabihf/9.3.0/../../../../arm-linux-gnueabihf/bin/ld: ../../libAlembic.so.1.7.12: undefined reference to `__atomic_compare_exchange_8'
/usr/lib/gcc/arm-linux-gnueabihf/9.3.0/../../../../arm-linux-gnueabihf/bin/ld: ../../libAlembic.so.1.7.12: undefined reference to `__atomic_load_8'
/usr/lib/gcc/arm-linux-gnueabihf/9.3.0/../../../../arm-linux-gnueabihf/bin/ld: ../../libAlembic.so.1.7.12: undefined reference to `__atomic_store_8'
```