Add BUILD_TESTING cmake option

As with CTest, but without the full machinery thereof.  This facilitates
package builders to use the usual build targets (all, install) without
needing to build the test programs if they're just going to get dropped
on the floor.

CMakeLists.txt

@@ -12,6 +12,9 @@ include(CheckIncludeFileCXX)
 include(CheckIPOSupported)
 include(CMakeDependentOption)
 
+# Name chosen for compatibility with CTest.
+option(BUILD_TESTING "Build test programs as well as shims" ON)
+
 option(SNMALLOC_HEADER_ONLY_LIBRARY "Use snmalloc has a header-only library" OFF)
 # Options that apply globally
 option(SNMALLOC_CI_BUILD "Disable features not sensible for CI" OFF)
@@ -373,10 +376,15 @@ if(NOT SNMALLOC_HEADER_ONLY_LIBRARY)
     target_compile_definitions(snmallocshim-checks-rust PRIVATE SNMALLOC_CHECK_CLIENT)
   endif()
 
-  enable_testing()
-
   set(TESTDIR ${CMAKE_CURRENT_SOURCE_DIR}/src/test)
-  subdirlist(TEST_CATEGORIES ${TESTDIR})
+
+  if(BUILD_TESTING)
+    enable_testing()
+    subdirlist(TEST_CATEGORIES ${TESTDIR})
+  else()
+    set(TEST_CATEGORIES "")
+  endif()
+
   list(REVERSE TEST_CATEGORIES)
   if (${SNMALLOC_CLEANUP} STREQUAL THREAD_CLEANUP)
     set(TEST_CLEANUP PTHREAD_DESTRUCTORS)