[CMake] Automatic out of source build's possible?

[Campbell Barton]

For blender we currently support 2 build systems - SCons and CMake,
Quite a few technical users build from source on *nix just to get the
latest version and use scons, I suspect this is because scons
configures every time and its simple just to type "scons" in the
source dir and end up with a build.
We have SCons configured to do an out-of-source build by default with
a predefined directory.

I wasn't aware of anything similar for CMake so I write a GNUmakefile
(included below) in the root source dir to do something similar for
CMake.
(note, we don't allow in-source builds at the moment so there is no
conflict with possible in-source makefiles).

This makefile creates a per-defined out-of-source build dir if
necessary and runs make, since we explicitly list source files and
headers in the CMakeLists.txt cmake will re-configure if needed.

So my questions are...
- Do other projects do this? is there some preferred way to do this?
- Is it possible to setup the CMakeLists.txt so the generated
makefiles are written to a directory other then the CWD?
- Is there any way to default to our-of-source build when running
"cmake ." in the source dir? (rather then aborting which is what we do
now).

Probably our users should just get the hang on setting up out of
source builds but I think they like the convenience.

# ------------
# This Makefile does an out-of-source CMake build in ../build/`OS`_`CPU`
# eg:
#   ../build/Linux_i386
# This is for users who like to configure & build blender with a single command.

# System Vars
OS:=$(shell uname -s)
OS_NCASE:=$(shell uname -s | tr '[A-Z]' '[a-z]')

# Source and Build DIR's
BLENDER_DIR:=$(shell pwd -P)
BUILD_DIR:=$(shell dirname $(BLENDER_DIR))/build/$(OS_NCASE)

# Get the number of cores for threaded build
NPROCS:=1
ifeq ($(OS), Linux)
    NPROCS:=$(shell grep -c ^processor /proc/cpuinfo)
endif
ifeq ($(OS), Darwin)
    NPROCS:=$(shell system_profiler | awk '/Number Of CPUs/{print $4}{next;}')
endif
ifeq ($(OS), FreeBSD)
    NPROCS:=$(shell sysctl -a | grep "hw.ncpu " | cut -d" " -f3 )
endif
ifeq ($(OS), NetBSD)
    NPROCS:=$(shell sysctl -a | grep "hw.ncpu " | cut -d" " -f3 )
endif


# Build Blender
all:
    @echo
    @echo Configuring Blender ...

    if test ! -f $(BUILD_DIR)/CMakeCache.txt ; then \
        mkdir -p $(BUILD_DIR) ; \
        cd $(BUILD_DIR) ; \
        cmake $(BLENDER_DIR) -DCMAKE_BUILD_TYPE:STRING=Release ; \
    fi

    @echo
    @echo Building Blender ...
    cd $(BUILD_DIR) ; make -s -j $(NPROCS)
    @echo
    @echo run blender from "$(BUILD_DIR)/bin/blender"
    @echo

.PHONY: all

# ------------

- Campbell