Add support for the Ninja CMake generator

[Manouchehri]

Using Ninja would significantly speed up the process of small rebuilds, and make parallelism work easily out of the box. This would help with #2 and #7.

Tried to do this myself, but wasn't able to figure out some of the CMakeLists.txt dependencies. Usually once I/we figure out what the build dependency issues are, it's as simple as:

cmake .. -GNinja -DCMAKE_INSTALL_PREFIX=/local/bin
ninja
ninja install

[stek29]

I doubt it would help much with #7 tbh :)

[tpruzina]

Hm, build seems to be able to saturate 16 threads (make -j16), don't think there is too much to gain there.
Switching to meson could somewhat speed up "./configure" phase, but considering the number of dependencies I don't think this is gonna be high on priority list.

As far as small rebuilds are concerned, I don't find them to be that terrible, just touched a few files in src/ and stuff builds in ~3-4 seconds.

make -j16 4.70s user 1.33s system 149% cpu 4.026 total

[s3rvac]

I tried to tackle this as I wanted to get more familiar with the Ninja build system. Some of our repositories are already buildable with Ninja, but there is a general problem with the way we link libraries built via external projects.

For each external project, we create a target that we can later use in target_link_libraries(). Both GNU Make and Visual Studio are fine with the fact that libraries coming from external projects do not exist when the build starts (they will exist once the external project is built). However, ninja fails with

ninja: error: '/path/to/libfoo.a', needed by 'target', missing and no known rule to make it

From what I have found, I believe that we will need to specify all the libraries in the BUILD_BYPRODUCTS option of ExternalProject_Add so that Ninja knows they will be built later.

[Manouchehri]

Thanks for looking into it. =)

I ran into the same library linking issues, but I thought it was my fault. (I did a "fix" like avast/llvm@294bfb1 too, but ended up breaking the normal build.)

Is add_dependencies alone not enough?

[s3rvac]

No, add_depedencies() only says "build this after you have built that". It does not say anything about the origin of /path/to/libfoo.a (that it is a product of a particular external project). Only after I added the library into BUILD_BYPRODUCTS, the Ninja build started working. I guess the Ninja generator is more strict than other generators.