FBFetchFromManifest: build fetched dependencies without warnings

Summary:
A project that fetches its dependencies with `fb_fetch_from_manifest()` compiles them with its own flags, so it prints every warning they have. Most of the EdenFS Linux GitHub workflow's 81,000-line Build step is warnings in fbthrift's, fb303's, wangle's, fizz's and edencommon's sources, about 2,200 of them with their notes. Fixing them is outside the scope of eden's open source build, and they bury eden's own warnings. Each dependency's own builds still report them:
https://github.com/meta/sapling/actions/runs/41234051864

This builds a fetched dependency with `-w`, so it prints no more than an installed copy would. MSVC would warn about `/w` overriding the dependency's own `/W` level, so this leaves MSVC builds alone. Eden's own warnings, including those in the dependencies' headers it includes, are unchanged.

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landed-with-radar-review

Differential Revision: D128861329

fbshipit-source-id: 7e629e26f3c6722036fe34380de3f3402e260243

Author: genevievehelsel

build/fbcode_builder/CMake/FBFetchFromManifest.cmake

@@ -138,6 +138,12 @@ function(fb_fetch_from_manifest name manifest)
   # Nor may its export(PACKAGE), as in cpptoml, put its build tree in the
   # user's package registry, where later configures would find it.
   set(CMAKE_EXPORT_NO_PACKAGE_REGISTRY ON)
+  # Its warnings are for its own project to fix, and an installed copy
+  # wouldn't print them either.
+  if(NOT MSVC)
+    string(APPEND CMAKE_C_FLAGS " -w")
+    string(APPEND CMAKE_CXX_FLAGS " -w")
+  endif()
   include(FetchContent)
   FetchContent_Declare(${name} ${source} ${ARGN} ${exclude})
   # CMAKE_REQUIRED_* configure this project's check_*() calls, and a