rts.cabal.in: advertise profiling flavours of libraries, behind a flag

The make build system appears to be doing a bit of magic in order to supply
the profiled flavours of libHSrts and libCffi, as they're not advertised in
the 'extra-library-flavours' field of rts.cabal.in. This patch explicitly
advertises _p and _thr_p flavours of the RTS library and libCffi, but only
when the RTS is configured with the (newly introduced) 'profiling' flag.

This is necessary for Hadrian, as a branch (soon to be merged) does away with
ghc-cabal and relies just on Cabal to get package information. Without this
patch, Cabal can never inform us that _p and _thr_p flavours should be
built (and registered in the package db) as well, which obviously prevents us
from building a profiled GHC.

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4409

rts/rts.cabal.in

@@ -26,6 +26,8 @@ flag leading-underscore
   default: @CabalLeadingUnderscore@
 flag smp
   default: True
+flag profiling
+  default: False
 
 library
     -- rts is a wired in package and
@@ -43,7 +45,16 @@ library
     -- libCffi_debug libCffi_ libCffi_l libCffi_p
     -- libCffi_thr libCffi_thr_debug libCffi_thr_l libCffi_thr_p
     extra-library-flavours: _debug _l _thr _thr_debug _thr_l
-    -- TODO: _p and _thr_p only if profiling.
+
+    -- The make build system seems to be doing something "magic"/special
+    -- for generating profiled builds of those libraries, but we need to
+    -- be transparent for hadrian which gets information about the rts
+    -- "package" through Cabal and this cabal file. We therefore declare
+    -- two profiling-enabled flavours to be available when passing the
+    -- 'profiling' flag when configuring the RTS from hadrian, using Cabal.
+    if flag(profiling)
+      extra-library-flavours: _p _thr_p
+
     exposed: True
     exposed-modules:
     if flag(libm)