Add BUILD_TESTS option to disable test suite

By default, PolarSSL builds a fairly extensive test suite to validate
the library. In the context of Buildroot, building this test suite is
not really useful, so we add a BUILD_TESTS to disable its build.

[Gustavo: update for 1.2.11]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

diff -Nura polarssl-1.2.11.orig/CMakeLists.txt polarssl-1.2.11/CMakeLists.txt
--- polarssl-1.2.11.orig/CMakeLists.txt	2014-07-11 17:14:43.414651327 -0300
+++ polarssl-1.2.11/CMakeLists.txt	2014-07-11 17:23:00.573498626 -0300
@@ -49,9 +49,11 @@
 add_subdirectory(library)
 add_subdirectory(include)
 
-if(CMAKE_COMPILER_IS_GNUCC)
+option(BUILD_TESTS "Build tests." ON)
+
+if(CMAKE_COMPILER_IS_GNUCC AND BUILD_TESTS)
   add_subdirectory(tests)
-endif(CMAKE_COMPILER_IS_GNUCC)
+endif(CMAKE_COMPILER_IS_GNUCC AND BUILD_TESTS)
 if(CMAKE_COMPILER_IS_CLANG)
   add_subdirectory(tests)
 endif(CMAKE_COMPILER_IS_CLANG)