libabsl_flags_usage.so does not have a SONAME

**Describe the bug**

libabsl_flags_usage.so lacks SONAME.

**Steps to reproduce the bug**

The toolchain complains while building or-tools:
> ====> Running Q/A tests (stage-qa)
> Error: /usr/local/bin/assignment_mip is linked to /usr/local/lib/libabsl_flags_usage.so which does not have a SONAME. devel/abseil needs to be fixed.

**What version of Abseil are you using?**
20200923.2

**What operating system and version are you using**

FreeBSD 12.2

**What compiler and version are you using?**

clang-10

**What build system are you using?**

cmake