Add option to build monolith (single library)

[jeroen]

Statically linking abseil is often difficult, because we need to link 20 or 30 libs, which change from version to version in abseil.

For example to link a grpc client I currently need this:

-labsl_raw_hash_set -labsl_hash -labsl_city -labsl_low_level_hash -labsl_random_internal_pool_urbg \
-labsl_random_internal_randen -labsl_random_internal_randen_hwaes -labsl_random_internal_randen_hwaes_impl \
-labsl_random_internal_randen_slow -labsl_random_internal_platform -labsl_random_internal_seed_material \
-labsl_random_seed_gen_exception -labsl_statusor -labsl_status -labsl_cord -labsl_cordz_info -labsl_cord_internal \
-labsl_cordz_functions -labsl_exponential_biased -labsl_cordz_handle -labsl_strerror -labsl_str_format_internal \
-labsl_synchronization -labsl_stacktrace -labsl_symbolize -labsl_malloc_internal -labsl_time  -labsl_strings \
-labsl_strings_internal -labsl_base -labsl_spinlock_wait -labsl_int128 -labsl_throw_delegate -labsl_time_zone \
-labsl_raw_logging_internal

The V8 project has a build option v8_monolithic. In this case the build will produce a single libv8_monolith.a file that contains the libs. This really simplifies things, as we can link to it as a simple single library.

It would be useful to have a similar option for abseil. So that we can choose to build a single library, that allows us to just do -labsl without worrying about the particular flags, and have the linker figure out which symbols it needs.