* Find and add abseil even when BUILD_GEOCODER=OFF

* Treat boolean variables as conditions

* Expose BUILD_SHARED_LIBS as a user configurable option

* Add BUILD_TESTING option

* Wrap test dependencies and targets in if (BUILD_TESTING)

* Refactor library build procedure to respect user defined configurations

* Export cmake package config files so downstream projects can use the library without creating a FindLibPhonenumber

* Bump version number to 8.13.0

Co-authored-by: penmetsaa <penmetsaa@google.com>

* Readme update w.r.t new abseil-cpp dependency and fixing the absl-cpp version that we download.

* Reducing hash of commit to just working level

Currently, CMake is configured to download and build abseil from source
whenever `BUILD_GEOCODER` is set to `ON`. Let's allow users to use a
pre-existing installation of abseil by checking for one first before
attempting to download and build another one.

If CMake finds a pre-existing installation of abseil, it will use that
instead.

We need to allow the user to override `CMAKE_CXX_STANDARD` because they
may have built abseil with a newer standard than C++11. Attempting to
build libphonenumber with C++11 will fail if abseil was built with a
newer C++ standard.

Closes #2772.

- Faced link errors stemmed from mismatch of api definitions of absl::node_hash_set. So explicitly mentioned this lib as dependency during linking. This is common case/solution for absl hash set lib's link errors as per community.
- Decided to continued to have wrapper (stringutil), so that in future it's easy if we decided to change implementation; client code is safe. Changed most of the implementations & data structures in stringutil, to use abseil stuffs more.
- Though absl::string_view is suitable for most cases where we want immutable/read-only string structures, limiting to stringutil apis, not changing public API signatures (even though no compliance issue); reason: reduce dependency on abseil libs.
- Abseil specific changes done:
    - std::set => absl::node_hash_set
    - std::map => absl::node_hash_map
    - Using absl alternatives for “SplitStringUsing” API and “StringHolder” class impl.
    - Replaced manual impl of prefix and suffix checks with absl::StartsWith() and absl::EndsWith.
    - Similarly used absl::StrReplaceAll, absl::StrCat, absl::StrAppend where possible; and for type conversion apis.
- Note: Tried applying parameter packs to replace manually written duplicate ```StrCat``` apis (for accommodating more number of parameters of same type), but it did not go well when different type arguments passed. Eg char, int types to be converted to StringHolder; faced constructor implicit conversion issues.

@API updates:
- Migrating from associative containers to more performant types. Eg: map to absl::btree_map.
- Synchronise the write access to map "AreaCodeMaps" using absl::Mutex locking api.
- Use more of absl:strings packages like absl::StrReplaceAll

@Build updates:
- To build (CMake) against absl packages, the minimum version of compiler is CPP11+, whereas earlier we are not mandating this.
- We are upgrading CMAKE version also to automate building the external absl packages.

The change is announced. Please report issues in case of any breakages.
https://issuetracker.google.com/issues?q=componentid:192347%20555

library but be able to build the library from source as current practice is.
Review URL: https://codereview.appspot.com/6602047