I tried building and installing the source code from git (I tried both master and tags/v7.0-beta1 with the same error) on Mac OS, installing as dependencies only wget/cmake (using homebrew).

Configuring, building and even testing using the standard CMake workflow

mkdir build && cd build
cmake ..
make -j8

work fine (except for tons of warnings regarding the deprecated "register" keyword and some .cc-files without symbols).

However, running make install (or, equivalently, cmake --build . --target install) fails because the file build/abseil-cpp-src/CMake/absl-config.cmake is missing.
I tried to run

find build | grep absl-config

too see whether the file is present somewhere else, but there appears to be no such file in the build directory.

-- Installing: /usr/local/lib/cmake/protobuf/protobuf-module.cmake
-- Installing: /usr/local/lib/cmake/protobuf/protobuf-options.cmake
-- Installing: /usr/local/lib/cmake/protobuf/protobuf-config.cmake
-- Installing: /usr/local/lib/cmake/protobuf/protobuf-config-version.cmake
-- Installing: /usr/local/lib/cmake/absl/absl-targets.cmake
-- Installing: /usr/local/lib/cmake/absl/absl-targets-release.cmake
CMake Error at abseil-cpp-build/cmake_install.cmake:55 (file):
  file INSTALL cannot find
  "/Users/me/Source/or-tools/build/abseil-cpp-src/CMake/absl-config.cmake".
Call Stack (most recent call first):
  external/cmake_install.cmake:41 (include)
  cmake_install.cmake:103 (include)

If I manually add an empty file in its place, the install goes through without error. However, as expected, I cannot use ortools in my cmake project with an empty absl-config.cmake file installed:

find_package(ortools CONFIG REQUIRED)

yields the following error:

CMake Error at CMakeLists.txt:12 (find_package):
  Found package configuration file:

    /usr/local/lib/cmake/ortools/ortoolsConfig.cmake

  but it set ortools_FOUND to FALSE so package "ortools" is considered to be
  NOT FOUND.  Reason given by package:

  The following imported targets are referenced, but are missing:
  absl::absl_base absl::absl_container absl::absl_hash absl::absl_memory
  absl::absl_meta absl::absl_str_format absl::absl_strings
  absl::absl_synchronization absl::absl_types

I then proceeded to find absl-config.cmake in /patches (in the source directory).
This file seems to contain a syntax error at least in tags/v7.0-beta1 (missing closing parenthesis).
I guess something was supposed to copy that file to the build directory where install expects it; I did that manually, retried the install, and it worked.

I still had no luck consuming ortools cmake config files in my cmake project, still with the same error message as before.
Adding a
find_package(absl CONFIG REQUIRED)
before that fixed this issue, so that configuring, building and linking (in my own CMake project) against ortools::ortools now works.

Was I doing something wrong here, like missing a "patching" step when running cmake? Then that should maybe be included somewhere in the documentation.