cmake: add ABSL_BUILD_TESTING option (#1057)

Abseil's own tests now are disabled if either BUILD_TESTING
or a new option called ABSL_BUILD_TESTING is false.
Additionally, Abseil's CMakeLists.txt no longer re-declares
the BUILD_TESTING option with a value of false.

Abseil had been using just the BUILD_TESTING option, since
the fix for #901.  Because setting BUILD_TESTING false still
works to disable Abseil's tests, this change preserves the
behavior asked for in that issue.

Previous to that, Abseil had a project specific flag for
this, as is the typical idiom used in other projects.

The issue with BUILD_TESTING is that it is an all-or-nothing
policy.  When Abseil is incorporated as a subproject, the
encompasing project has no convenient way to enable its own
tests while disabling Abseil's.

Fixes #1056

3 files changed, 13 insertions, 9 deletions

diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 17c4f449..9ad2627d 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -40,7 +40,8 @@ endif()
 # LINKOPTS: List of link options
 # PUBLIC: Add this so that this library will be exported under absl::
 # Also in IDE, target will appear in Abseil folder while non PUBLIC will be in Abseil/internal.
-# TESTONLY: When added, this target will only be built if BUILD_TESTING=ON.
+# TESTONLY: When added, this target will only be built if both
+#           BUILD_TESTING=ON and ABSL_BUILD_TESTING=ON.
 #
 # Note:
 # By default, absl_cc_library will always create a library named absl_${NAME},
@@ -82,7 +83,7 @@ function(absl_cc_library)
     ${ARGN}
   )
 
-  if(ABSL_CC_LIB_TESTONLY AND NOT BUILD_TESTING)
+  if(ABSL_CC_LIB_TESTONLY AND NOT (BUILD_TESTING AND ABSL_BUILD_TESTING))
     return()
   endif()
 
@@ -364,7 +365,7 @@ endfunction()
 #     GTest::gtest_main
 # )
 function(absl_cc_test)
-  if(NOT BUILD_TESTING)
+  if(NOT (BUILD_TESTING AND ABSL_BUILD_TESTING))
     return()
   endif()
 
diff --git a/CMake/README.md b/CMake/README.md
index f8b27e63..8134615e 100644
--- a/CMake/README.md
+++ b/CMake/README.md
@@ -20,8 +20,10 @@ googletest framework
 ### Step-by-Step Instructions
 
 1. If you want to build the Abseil tests, integrate the Abseil dependency
-[Google Test](https://github.com/google/googletest) into your CMake project. To disable Abseil tests, you have to pass
-`-DBUILD_TESTING=OFF` when configuring your project with CMake.
+[Google Test](https://github.com/google/googletest) into your CMake
+project. To disable Abseil tests, you have to pass either
+`-DBUILD_TESTING=OFF` or `-DABSL_BUILD_TESTING=OFF` when configuring your
+project with CMake.
 
 2. Download Abseil and copy it into a subdirectory in your CMake project or add
 Abseil as a [git submodule](https://git-scm.com/docs/git-submodule) in your
@@ -91,7 +93,8 @@ setting a consistent `CMAKE_CXX_STANDARD` that is sufficiently high.
 
 ### Running Abseil Tests with CMake
 
-Use the `-DBUILD_TESTING=ON` flag to run Abseil tests.
+Use the `-DABSL_BUILD_TESTING=ON` flag to run Abseil tests.  Note that
+BUILD_TESTING must also be on (the default).
 
 You will need to provide Abseil with a Googletest dependency.  There are two
 options for how to do this:
@@ -109,7 +112,7 @@ For example, to run just the Abseil tests, you could use this script:
 cd path/to/abseil-cpp
 mkdir build
 cd build
-cmake -DBUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON ..
+cmake -DABSL_BUILD_TESTING=ON -DABSL_USE_GOOGLETEST_HEAD=ON ..
 make -j
 ctest
 ```
@@ -175,7 +178,7 @@ cmake --build /temporary/build/abseil-cpp --target install
 
 ## Google Test Options
 
-`-DBUILD_TESTING=ON` must be set to enable testing
+`-DABSL_BUILD_TESTING=ON` must be set to enable testing
 
 - Have Abseil download and build Google Test for you: `-DABSL_USE_EXTERNAL_GOOGLETEST=OFF` (default)
   - Download and build latest Google Test: `-DABSL_USE_GOOGLETEST_HEAD=ON`
diff --git a/CMake/install_test_project/test.sh b/CMake/install_test_project/test.sh
index aecbb8fe..cc028bac 100755
--- a/CMake/install_test_project/test.sh
+++ b/CMake/install_test_project/test.sh
@@ -55,7 +55,7 @@ cmake "${absl_dir}" \
   -DABSL_USE_EXTERNAL_GOOGLETEST=ON \
   -DABSL_FIND_GOOGLETEST=ON  \
   -DCMAKE_BUILD_TYPE=Release \
-  -DBUILD_TESTING=ON \
+  -DABSL_BUILD_TESTING=ON \
   -DBUILD_SHARED_LIBS="${build_shared_libs}"
 make -j $(nproc)
 ctest -j $(nproc) --output-on-failure