cmake: use -Wall on GCC and Clang

These checks come from [1]. The `MATCHES` operator is used for clang
because of Apple's special string.

[1] http://stackoverflow.com/questions/10046114/in-cmake-how-can-i-test-if-the-compiler-is-clang/10055571#10055571

CMakeLists.txt

@@ -10,6 +10,10 @@ include(CTest)
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
+if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    set(CMAKE_CXX_FLAGS "-Wall ${CMAKE_CXX_FLAGS}")
+endif()
+
 add_library(spdlog INTERFACE)
 
 option(SPDLOG_BUILD_EXAMPLES "Build examples" OFF)