boost manifest: build from source on Linux with -fPIC

Summary:
Drop the [debs] satisfier (libboost-all-dev) and add cxxflags/cflags=-fPIC
under [b2.args.os=linux] so that boost's static archives are PIC and can
be linked into downstream shared libraries (specifically rebalancer's
librebalancer.so).

The Ubuntu libboost-all-dev package ships static archives built without
-fPIC, which fails at link time with R_X86_64_PC32 / 'recompile with
-fPIC' errors when the consumer is a shared library. Building boost
ourselves with cflags=-fPIC fixes this. Boost's b2 build does not pick
PIC up from CMAKE_POSITION_INDEPENDENT_CODE because it is not CMake-driven.

Note: this changes behavior for every consumer of fbcode_builder that
runs with --allow-system-packages on Debian/Ubuntu — they will now build
boost from source instead of installing libboost-all-dev. Build time for
boost is ~5-10 min on a 16-core runner. If this is unacceptable for any
consumer, the right follow-up is a per-consumer way to keep the system
satisfier (e.g. opt-in [debs] block keyed off a context flag).

Reviewed By: bigfootjon

Differential Revision: D103767938

fbshipit-source-id: 06275b77b8635f1a5742e1289e0ecc6e762e70c7

Author: r-barnes

build/fbcode_builder/manifests/boost

@@ -14,8 +14,9 @@ sha256 = c86bd9d9eef795b4b0d3802279419fde5221922805b073b9bd822edecb1ca28e
 BOOST_ROOT_1_69_0
 BOOST_ROOT_1_83_0
 
-[debs]
-libboost-all-dev
+# Build boost from source rather than satisfying it from libboost-all-dev.
+# The Ubuntu boost packages ship static archives without -fPIC, which
+# fails to link into our shared rebalancer library.
 
 [homebrew]
 boost
@@ -106,6 +107,12 @@ patchfile = boost_1_83_0.patch
 # RHEL hardened gcc is not compatible with PCH
 # https://bugzilla.redhat.com/show_bug.cgi?id=1806545
 pch=off
+# Build with -fPIC so the static archives we install can be linked into
+# downstream shared libraries (e.g. rebalancer's librebalancer.so).
+# Boost's b2 build does not pick this up from CMAKE_POSITION_INDEPENDENT_CODE
+# because it is not driven by CMake.
+cxxflags=-fPIC
+cflags=-fPIC
 
 [b2.args.os=darwin]
 toolset=clang