build: minor tweaks for speeding up the configure phase

Only the C language is used unless we are building the ASAN or UBSAN
build variants. Only check for the C language support, enabling C++
support in the case that the build type is one of those two.

Restructure the file layout to place the custom modules into the
`cmake/modules` directory that is the common layout for CMake based
projects.

While a generally unimportant optimization, it makes it much more clear
what is being checked and why and does reduce the configure time by ~10%
(.25s) on Windows.

CMakeLists.txt

@@ -7,9 +7,15 @@ if(POLICY CMP0092)
   cmake_policy(SET CMP0092 NEW)
 endif()
 
-project(cmark VERSION 0.30.3)
+project(cmark
+  LANGUAGES C
+  VERSION 0.30.3)
 
-include("FindAsan.cmake")
+if(CMAKE_BUILD_TYPE MATCHES "asan|ubsan")
+  enable_language(CXX)
+  list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules)
+  include(FindAsan)
+endif()
 include(GNUInstallDirs)
 include(CTest)