Update toolchains we use for CI

tools/gcc-8.dds

@@ -0,0 +1,5 @@
+Compiler-ID: GNU
+C++-Version: C++17
+C++-Compiler: g++-8
+Flags: -fconcepts -DFMT_HEADER_ONLY=1
+Optimize: True

tools/msvc.dds

@@ -1,4 +1,5 @@
 Compiler-ID: MSVC
 C++-Version: C++17
-Flags: /experimental:preprocessor
+Flags: /experimental:preprocessor /D FMT_HEADER_ONLY=1
 Link-Flags: rpcrt4.lib
+Optimize: True