[LLVMdev] Add ability for external projects use for build installed version LLVM [PATCH]

[Vladimir Merzliakov]

> Vladimir,
>
> I took a *very quick* look at your patch. I won't be able to look at it in
> more detail until this weekend, but perhaps you can address a couple of
> concerns:
>
> 1. I don't understand the need for LLVMToolDirSlash. This seems like a
>    gratuitous change to me.
If LLVM_OBJ_ROOT (with LLVM_SRC_ROOT) and LLVM_ROOT both not set
without LLVMToolDirSlash we will have "/gccas$(EXEEXT)", for example.
This is not problem for project that not use LLVM tools and libs.
But this not permite one useful case: project that use only LLVM tools and 
LLVM tools
directory added to PATH.
In this case using LLVMToolDirSlash we have correct tool name
"gccas$(EXEEXT)" and tool can find by PATHs.

> 2. I don't think the change to CPP.Flags is right. It won't find the LLVM
>    header files
I build my frontend in LLVM_ROOT set case and LLVM headers find fine.

I already found bug in LLVM_OBJ_ROOT case in proposed patch :(
+  LLVMIncDirs := $(LLVM_OBJ_ROOT)/$(BuildMode)/include
instead

+  LLVMIncDirs := $(LLVM_OBJ_ROOT)/include

> 3. Removing the .PRECIOUS line will interfere with correct operation of
>    parallel (make -j N) builds.
When external project build
$(LLVMLibDir)/.dir $(LLVMToolDir)/.dir $(LLVMExmplDir)/.dir files and
directories already exist and then
using .PRECIOUS redundent in this case (as i think).
In time LLVM build this files and directories listed in line:
.PRECIOUS: $(ObjDir)/.dir $(LibDir)/.dir $(ToolDir)/.dir $(ExmplDir)/.dir
I will test patch with parallel builds.

Related question lines:
-LLVMExmplDir:= $(LLVM_OBJ_ROOT)/$(BuildMode)/examples
and
-.PRECIOUS: $(LLVMLibDir)/.dir $(LLVMToolDir)/.dir $(LLVMExmplDir)/.dir

Is removing LLVMExmplDir definition is ok? It not used anywhere in LLVM CVS

> 4. I don't understand why LLVMUsedLibs and other variables should only be
>    set if LLVMLibDir is defined.
I don link strange filenames if LLVMLibDir will not define and  project
tooldir/Makefile set LLVMLIBS.
But i will skip this part of changes in next version of patch.

> Before this change is committed, you need to test that all our scenarios,
> in each combination:
>
> 1. Both with and without OBJ_DIR == SRC_DIR
> 2. Single and Parallel builds
> 3. Building LLVM versus building a project (and now building from
> installed)
ok, will do...

I found some bugs in my patch and then I will send updated version after it 
testing.

Thank you for comments!

Vladimir