Get closer to true out-of-source builds for Hack

Summary:
It should be possible to build HHVM and Hack from an entirely different directory
to the source, and for the source directory to be completely unchanged.

This is expected by Debian build scripts, for example.

This gets us /almost/ all the way there - but Dune still generates `.merlin`
files in the source directory. ocaml/dune#2568 to address that.

Reviewed By: jjergus

Differential Revision: D16944399

fbshipit-source-id: b0b4fc17870dfda95d3d555928a0c99911fdeee0

hphp/hack/CMakeLists.txt

@@ -33,7 +33,7 @@ if(NOT MAKE)
   set(MAKE make)
 endif()
 
-if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_CURRENT_BINARY_DIR)
+if ("${CMAKE_CURRENT_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_BINARY_DIR}")
   set(DUNE_BUILD_DIR "${CMAKE_CURRENT_SOURCE_DIR}/_build")
   set(OPAM_STAMP_FILE "_build/opam.stamp")
 else()

hphp/hack/scripts/build_rust_to_ocaml.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 set -e
-HACK_SOURCE_ROOT=${HACK_SOURCE_ROOT:-$HACKDIR}
+HACK_SOURCE_ROOT="${HACK_SOURCE_ROOT:-$HACKDIR}"
+HACK_BUILD_ROOT="${HACK_BUILD_ROOT:-$HACKDIR}"
 
 if [ -z "$HACK_SOURCE_ROOT" ]; then
    echo >&2 "ERROR: must set HACK_SOURCE_ROOT to point to hphp/hack source dir"
@@ -21,6 +22,8 @@ else
   LOCK_FLAG="--locked"
 fi
 
+TARGET_DIR="${HACK_BUILD_ROOT}/target"
+
 profile=debug; profile_flags=
 if [ -z ${HACKDEBUG+1} ]; then
   profile=release; profile_flags="--release"
@@ -29,5 +32,10 @@ fi
   [[ -n "$CARGO_BIN" ]] && PATH="$CARGO_BIN:$PATH";
   # note: --manifest-path doesn't work with custom toolchain, so do cd
   cd "$HACK_SOURCE_ROOT" && \
-  cargo build $LOCK_FLAG --package "$pkg" $profile_flags "$@"
-) && cp "$HACK_SOURCE_ROOT/target/$profile/lib$lib.a" "lib${lib}_stubs.a"
+  cargo build \
+    $LOCK_FLAG \
+    --target-dir "${TARGET_DIR}" \
+    --package "$pkg" \
+    $profile_flags \
+    "$@"
+) && cp "${TARGET_DIR}/$profile/lib$lib.a" "lib${lib}_stubs.a"