cmake: fix CMakeLists.txt on Linux

CMakeLists.txt didn't follow the grammar of `set`, and it will fail when
setting `USE_VCPKG` off on non-Windows platforms.

When the platform is Linux, the Makefile adds `compat/linux/procinfo.o`
to `COMPAT_OBJS`, but the CMakeLists.txt didn't add
`compat/linux/procinfo.c` to `compat_SOURCES`. It would cause linkage
error.

Signed-off-by: Yuyi Wang <Strawberry_Str@hotmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/buildsystems/CMakeLists.txt

@@ -54,7 +54,7 @@ set(CMAKE_SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/../..)
 
 option(USE_VCPKG "Whether or not to use vcpkg for obtaining dependencies.  Only applicable to Windows platforms" ON)
 if(NOT WIN32)
-	set(USE_VCPKG OFF CACHE BOOL FORCE)
+	set(USE_VCPKG OFF CACHE BOOL "" FORCE)
 endif()
 
 if(NOT DEFINED CMAKE_EXPORT_COMPILE_COMMANDS)
@@ -277,7 +277,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
 
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
 	add_compile_definitions(PROCFS_EXECUTABLE_PATH="/proc/self/exe" HAVE_DEV_TTY )
-	list(APPEND compat_SOURCES unix-socket.c unix-stream-server.c)
+	list(APPEND compat_SOURCES unix-socket.c unix-stream-server.c compat/linux/procinfo.c)
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")