CMake: Only build unit tests if Mbed OS is the current project

Typically when adding a unit test directory to a CMake project a check
will be used to ensure the subdirectory is added only if the following
are true:

* The BUILD_TESTING option is set to ON.
* The current CMake project is the top-level project.

The reason being, if a downstream project includes our project they
generally don't want to build our unit tests.

In mbed-os, we do correctly specify the above condition before adding
the central UNITTEST subdirectory, which fetches googletest and adds the
"stub" libraries the unit tests depend on. However, we only check if
CMAKE_CROSSCOMPILING is OFF (or undefined) before actually adding the
unit tests. This mismatched logic would lead to unexpected build
failures in various scenarios. One likely case could be: a downstream
project including mbed-os happens to set CMAKE_CROSSCOMPILING to
OFF/undefined for any reason (possibly to build its own unit tests).
mbed-os would go ahead and attempt to build its tests without fetching
googletest or adding the required stub targets.

To fix the issue replace the check for CMAKE_CROSSCOMPILING in the unit
tests with the same BUILD_TESTING idiom we use for adding the central
UNITTESTS subdirectory.

connectivity/cellular/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

connectivity/lorawan/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

connectivity/netsocket/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

drivers/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

events/CMakeLists.txt

@@ -1,9 +1,10 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 else()
+
 add_library(mbed-events INTERFACE)
 
 target_include_directories(mbed-events

platform/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

storage/blockdevice/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

storage/kvstore/filesystemstore/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()
 

storage/kvstore/tdbstore/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if(NOT ${CMAKE_CROSSCOMPILING})
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     add_subdirectory(tests/UNITTESTS)
 endif()