Remove a CMake 3.19 construct

CONFIG supports multiple arguments only as of CMake 3.19, see
https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html#configuration-expressions

Ubuntu focal ships with 3.16 by default, so enable building with that.
Note that there is also the workaround of installing "cmake-mozilla".

CMakeLists.txt

@@ -56,7 +56,8 @@ function(CREATE_TARGET target)
         env ${VARS_FOR_CARGO}
           ${Rust_CARGO}
             build --bin ${target}
-            $<$<CONFIG:Release,RelWithDebInfo>:--release>
+            $<$<CONFIG:Release>:--release>
+            $<$<CONFIG:RelWithDebInfo>:--release>
             --target ${Rust_CARGO_TARGET}
             ${CARGO_FLAGS}
             ${FEATURES_ARG}

cmake/Rust.cmake

@@ -19,7 +19,7 @@ else()
 endif()
 
 set(rust_profile $<IF:$<CONFIG:Debug>,debug,release>)
-set(rust_debugflags "$<$<CONFIG:Debug,RelWithDebInfo>:-g>")
+set(rust_debugflags "$<$<CONFIG:Debug>:-g>$<$<CONFIG:RelWithDebInfo>:-g>")
 
 
 # Temporary hack to propogate CMake flags/options to build.rs. We need to get CMake to evaluate the