build: simplify the default calculation for SwiftSyntax

`CMAKE_SYSTEM_NAME` is **always** available - it is a CMake intrinsic
variable, the precondition is meaningless.

SwiftSyntax and SwiftSyntaxParser only require clang and libdispatch.
Merge the condition checks to just that.

Author: compnerd

CMakeLists.txt

@@ -409,30 +409,13 @@ if(MSVC OR "${CMAKE_SIMULATE_ID}" STREQUAL MSVC)
   include(ClangClCompileRules)
 endif()
 
-precondition(CMAKE_SYSTEM_NAME)
-if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+if(CMAKE_SYSTEM_NAME STREQUAL Darwin OR
+    EXISTS ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE})
   set(SWIFT_BUILD_SYNTAXPARSERLIB_default TRUE)
   set(SWIFT_BUILD_SOURCEKIT_default TRUE)
-elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
-  if(EXISTS ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE})
-    set(SWIFT_BUILD_SYNTAXPARSERLIB_default TRUE)
-    set(SWIFT_BUILD_SOURCEKIT_default TRUE)
-  else()
-    set(SWIFT_BUILD_SYNTAXPARSERLIB_default FALSE)
-    set(SWIFT_BUILD_SOURCEKIT_default FALSE)
-  endif()
-elseif(CMAKE_SYSTEM_NAME STREQUAL Windows)
-  if(EXISTS ${SWIFT_PATH_TO_LIBDISPATCH_SOURCE} AND
-     CMAKE_CXX_COMPILER_ID STREQUAL Clang)
-    set(SWIFT_BUILD_SYNTAXPARSERLIB_default TRUE)
-    set(SWIFT_BUILD_SOURCEKIT_default TRUE)
-  else()
-    set(SWIFT_BUILD_SYNTAXPARSERLIB_default FALSE)
-    set(SWIFT_BUILD_SOURCEKIT_default FALSE)
-  endif()
 else()
-    set(SWIFT_BUILD_SYNTAXPARSERLIB_default FALSE)
-    set(SWIFT_BUILD_SOURCEKIT_default FALSE)
+  set(SWIFT_BUILD_SYNTAXPARSERLIB_default FALSE)
+  set(SWIFT_BUILD_SOURCEKIT_default FALSE)
 endif()
 option(SWIFT_BUILD_SYNTAXPARSERLIB
     "Build the Swift Syntax Parser library"