core: make cpputest optional in cmake compilation

With this fix, WeeChat can be built without tests (if cpputest is not found),
without having to explicitely disable tests ("-DENABLE_TESTS=OFF").

CMakeLists.txt

@@ -212,7 +212,8 @@ endif()
 add_subdirectory(src)
 add_subdirectory(doc)
 
-if(ENABLE_TESTS)
+find_package(CppUTest)
+if(ENABLE_TESTS AND CPPUTEST_FOUND)
   enable_testing()
   add_subdirectory(tests)
 endif()

cmake/FindCppUTest.cmake

@@ -33,5 +33,5 @@ endif()
 
 find_package(PkgConfig)
 if(PKG_CONFIG_FOUND)
-  pkg_search_module(CPPUTEST REQUIRED cpputest)
+  pkg_search_module(CPPUTEST cpputest)
 endif()

tests/CMakeLists.txt

@@ -19,8 +19,6 @@
 
 enable_language(CXX)
 
-find_package(CppUTest REQUIRED)
-
 # fake ncurses lib (it does nothing)
 set(LIB_WEECHAT_NCURSES_FAKE_SRC ncurses-fake.c)
 add_library(weechat_ncurses_fake STATIC ${LIB_WEECHAT_NCURSES_FAKE_SRC})