Introduce CLAD_DISABLE_TESTS flag to build system

The testing and benchmarking relies on a hack that temporariliy changes
the compiler flags. This doesn't work in all environments, and it would
be nice if the the tests could be disabled so it's possible to build
Clad without this hack.

This makes it possible to build Clad on NixOS.

CMakeLists.txt

@@ -324,28 +324,34 @@ if (NOT CLAD_BUILD_STATIC_ONLY)
   add_subdirectory(demos/ErrorEstimation/CustomModel)
   add_subdirectory(demos/ErrorEstimation/PrintModel)
 
-  # Change the default compiler to the clang which we run clad upon. Our unittests
-  # need to use a supported by clad compiler. Note that's a huge hack and it is
-  # not guaranteed to work with cmake.
-  set(stored_cxx_compiler ${CMAKE_CXX_COMPILER})
-  set(stored_cxx_flags ${CMAKE_CXX_FLAGS})
-
-  set(CMAKE_CXX_COMPILER ${LLVM_TOOLS_BINARY_DIR}/clang)
-  # Filter some unsupported flags by clang.
-  string(REPLACE "-fno-lifetime-dse" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-  string(REPLACE "-Wno-class-memaccess" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+  if (NOT CLAD_DISABLE_TESTS OR CLAD_ENABLE_BENCHMARKS)
+    # Change the default compiler to the clang which we run clad upon. Our unittests
+    # need to use a supported by clad compiler. Note that's a huge hack and it is
+    # not guaranteed to work with cmake.
+    set(stored_cxx_compiler ${CMAKE_CXX_COMPILER})
+    set(stored_cxx_flags ${CMAKE_CXX_FLAGS})
+
+    set(CMAKE_CXX_COMPILER ${LLVM_TOOLS_BINARY_DIR}/clang)
+    # Filter some unsupported flags by clang.
+    string(REPLACE "-fno-lifetime-dse" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+    string(REPLACE "-Wno-class-memaccess" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+  endif()
 
-  add_subdirectory(unittests)
-  add_subdirectory(test)
+  if (NOT CLAD_DISABLE_TESTS)
+    add_subdirectory(unittests)
+    add_subdirectory(test)
+  endif()
 
   # Add benchmarking infrastructure.
   if (CLAD_ENABLE_BENCHMARKS)
     add_subdirectory(benchmark)
   endif(CLAD_ENABLE_BENCHMARKS)
 
-  # Restore the default compiler.
-  set(CMAKE_CXX_COMPILER ${stored_cxx_compiler})
-  set(CMAKE_CXX_FLAGS ${stored_cxx_flags})
+  if (stored_cxx_compiler)
+    # Restore the default compiler.
+    set(CMAKE_CXX_COMPILER ${stored_cxx_compiler})
+    set(CMAKE_CXX_FLAGS ${stored_cxx_flags})
+  endif()
 endif()
 
 # Workaround for MSVS10 to avoid the Dialog Hell