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.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: polarssl-1.1.1/CMakeLists.txt
===================================================================
--- polarssl-1.1.1.orig/CMakeLists.txt
+++ polarssl-1.1.1/CMakeLists.txt
@@ -27,9 +27,11 @@
 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)
 
 add_subdirectory(programs)