Clean up and document project files (#4560)

cmake/clang_flags.cmake

@@ -0,0 +1,20 @@
+# Ignored Clang warnings:
+# -Wno-c++98-compat               The library targets C++11.
+# -Wno-c++98-compat-pedantic      The library targets C++11.
+# -Wno-deprecated-declarations    The library contains annotations for deprecated functions.
+# -Wno-extra-semi-stmt            The library uses assert which triggers this warning.
+# -Wno-padded                     We do not care about padding warnings.
+# -Wno-covered-switch-default     All switches list all cases and a default case.
+# -Wno-unsafe-buffer-usage        Otherwise Doctest would not compile.
+
+set(CLANG_CXXFLAGS
+    -Werror
+    -Weverything
+    -Wno-c++98-compat
+    -Wno-c++98-compat-pedantic
+    -Wno-deprecated-declarations
+    -Wno-extra-semi-stmt
+    -Wno-padded
+    -Wno-covered-switch-default
+    -Wno-unsafe-buffer-usage
+)