Add FBGenCMakeBuildInfo.cmake and getdeps manifest

Summary:
Add the build infrastructure files needed for the BGP++ OSS CMake build system:

1. `FBGenCMakeBuildInfo.cmake` — copied from Open/R, provides `add_build_info()` and `generate_build_info()` CMake functions that auto-generate `CMakeBuildInfo.cpp` at build time. This populates the `BuildInfo_k*` constants declared in `bgp/cpp/common/CMakeBuildInfo.h`, which the OSS `BuildInfo.cpp` reads.

2. `opensource/fbcode_builder/manifests/bgppp` — getdeps.py manifest that declares BGP++ dependencies (folly, fbthrift, fb303, boost, re2), sets the CMake builder, and defines the ShipIt path mapping (`public_tld = .` maps to OSS repo root).

getdeps.py is Meta's OSS dependency builder. It reads this manifest, recursively builds all dependencies from source (folly → fbthrift → fb303), then invokes CMake for BGP++ with the correct CMAKE_PREFIX_PATH so find_package() resolves.

Differential Revision: D103337709

fbshipit-source-id: cd79448cc70ab18fd4e5635bfb290cd37e9c3e17

Author: induvsuresh

build/fbcode_builder/manifests/bgp

@@ -0,0 +1,26 @@
+[manifest]
+name = bgp
+fbsource_path = facebook/neteng/fboss/bgp
+shipit_project = bgp
+shipit_fbcode_builder = true
+
+[git]
+repo_url = https://github.com/facebook/BGP.git
+
+[build.os=linux]
+builder = cmake
+job_weight_mib = 3072
+
+[build.not(os=linux)]
+builder = nop
+
+[dependencies]
+boost
+fb303
+fbthrift
+folly
+re2
+
+[shipit.pathmap]
+fbcode/neteng/fboss/bgp = neteng/fboss/bgp
+fbcode/neteng/fboss/bgp/public_tld = .