Don't rely on CMake 3.0 features

Too recent (about one year).

CMakeLists.txt

@@ -40,10 +40,14 @@ option(USE_PKCS11_HELPER_LIBRARY "Build mbed TLS with the pkcs11-helper library.
 
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)
-option(ENABLE_TESTING "Build mbed TLS tests." ON)
 
-if(ENABLE_TESTING)
-    enable_testing()
+# Tests need Perl. Ideally we would use CMake's FindPerl module,
+# but it is only available since CMake 3.0 (released June 2014).
+# Assume Perl is available on Unix, and allow user to override.
+if(UNIX)
+    option(ENABLE_TESTING "Build mbed TLS tests." ON)
+else()
+    option(ENABLE_TESTING "Build mbed TLS tests." OFF)
 endif()
 
 if(LIB_INSTALL_DIR)
@@ -73,12 +77,13 @@ ADD_CUSTOM_TARGET(apidoc
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 
 if(ENABLE_TESTING)
-    find_package(PERL)
+    enable_testing()
 
-    if(PERL_FOUND)
     add_subdirectory(tests)
 
+    # additional convenience targets for Unix only
     if(UNIX)
+
         ADD_CUSTOM_TARGET(covtest
             COMMAND make test
             COMMAND programs/test/selftest
@@ -105,5 +110,4 @@ if(ENABLE_TESTING)
             COMMAND mv DartConfiguration.tcl.bak DartConfiguration.tcl
         )
     endif(UNIX)
-    endif(PERL_FOUND)
 endif()