Turn on warning as error only for Debug build

See #143:

> This project is widely packaged in Linux distros. Some of those distros aggressively adopt new compiler versions and therefore will see build warnings that aren't yet seen in your environment.

Author: xuhdev

src/CMakeLists.txt

@@ -71,9 +71,15 @@ endif()
 
 # Use high warning levels
 if(MSVC)
-  add_compile_options(/W4 /WX)
+    add_compile_options(/W4)
+    if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+        add_compile_options(/WX)
+    endif()
 else()
-  add_compile_options(-Wall -Wextra -Wpedantic -Werror)
+    add_compile_options(-Wall -Wextra -Wpedantic)
+    if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+        add_compile_options(-Werror)
+    endif()
 endif()
 
 add_subdirectory(lib)