common.cmake: downgrade C++ standard version

I was testing the new version (C++20) with some existing code, and
immediately got build errors.

Apparently, now this:

    const auto s = u8"hello";

means that s has the type of const char8_t *const, where char8_t is some
batshit insanity with 0 backwards compatibility. For example, you cannot
make a std::string out of this??? Fuck that, and fuck C++ committee
morons.

common.cmake

@@ -10,7 +10,7 @@
 # Add this to the top-level CMakeLists.txt (unless a higher version has already
 # been specified):
 #
-#     cmake_minimum_required(VERSION 3.12)
+#     cmake_minimum_required(VERSION 3.8)
 
 # Without this policy set, this line:
 #
@@ -54,7 +54,7 @@ if(parent_dir)
 endif()
 
 if(NOT DEFINED CC_CXX_STANDARD)
-    set(CC_CXX_STANDARD "20" CACHE STRING "C++ standard version")
+    set(CC_CXX_STANDARD "17" CACHE STRING "C++ standard version")
 endif()
 if(NOT DEFINED CC_COMMON_OPTIONS)
     option(CC_COMMON_OPTIONS "Set common compiler options" "${default_value}")

examples/boost/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.8)
 
 project(example_boost)
 

examples/dynamic/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.8)
 
 project(example_dynamic)
 

examples/simple/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.8)
 
 project(example_simple)
 

examples/static/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.8)
 
 project(example_static)