Ninja: Fix clang-cl /showIncludes prefix detection

We run MSVC-like compilers with the `/showIncludes` option and match the
output to extract the corresponding message.  Fix the matching to
support compilers like `clang-cl` that print the message on the first
line such that it is not preceded by a newline.

LLVM-Issue: https://llvm.org/bugs/show_bug.cgi?id=27226

Modules/CMakeDetermineCompilerId.cmake

@@ -664,8 +664,8 @@ function(CMAKE_DETERMINE_MSVC_SHOWINCLUDES_PREFIX lang)
     ERROR_VARIABLE err
     RESULT_VARIABLE res
     )
-  if(res EQUAL 0 AND "${out}" MATCHES "\n([^:]*:[^:]*:[ \t]*)")
-    set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "${CMAKE_MATCH_1}" PARENT_SCOPE)
+  if(res EQUAL 0 AND "${out}" MATCHES "(^|\n)([^:\n]*:[^:\n]*:[ \t]*)")
+    set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "${CMAKE_MATCH_2}" PARENT_SCOPE)
   else()
     set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "" PARENT_SCOPE)
   endif()