Uses/cmake.mk: Always pass FETCHCONTENT_FULLY_DISCONNECTED=1 to CMAKE…

…_ARGS.

Summary:
This variable makes `FetchContent_*()` CMake functions to disable any network
access, which is exactly what we expect in Ports framework.

Test Plan: `poudriere testport` on a simple CMake port.

Reviewed By: #kde, #portmgr, tcberner

Differential Revision: https://reviews.freebsd.org/D37015

Mk/Uses/cmake.mk

@@ -90,7 +90,8 @@ CMAKE_ARGS+=		-DCMAKE_C_COMPILER:STRING="${CC}" \
 			-DCMAKE_BUILD_TYPE:STRING="${CMAKE_BUILD_TYPE}" \
 			-DTHREADS_HAVE_PTHREAD_ARG:BOOL=YES \
 			-DCMAKE_INSTALL_RPATH_USE_LINK_PATH=YES \
-			-DCMAKE_VERBOSE_MAKEFILE:BOOL=ON
+			-DCMAKE_VERBOSE_MAKEFILE:BOOL=ON \
+			-DFETCHCONTENT_FULLY_DISCONNECTED:BOOL=ON
 
 # Handle the option-like CMAKE_ON and CMAKE_OFF lists.
 .  for _bool_kind in ON OFF