cmake (Windows): complain when encountering an unknown compiler

We have some custom handling regarding the link options, which are
specific to each compiler.

Therefore: let's not just continue without setting the link options if
configuring for a currently unhandled compiler, but error out.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/buildsystems/CMakeLists.txt

@@ -617,6 +617,8 @@ if(WIN32)
 		target_link_options(common-main PUBLIC -municode -Wl,-nxcompat -Wl,-dynamicbase -Wl,-entry:wmainCRTStartup -Wl,invalidcontinue.obj)
 	elseif(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
 		target_link_options(common-main PUBLIC /IGNORE:4217 /IGNORE:4049 /NOLOGO /ENTRY:wmainCRTStartup /SUBSYSTEM:CONSOLE invalidcontinue.obj)
+	else()
+		message(FATAL_ERROR "Unhandled compiler: ${CMAKE_C_COMPILER_ID}")
 	endif()
 elseif(UNIX)
 	target_link_libraries(common-main pthread rt)