cmake build does not support Ninja builds

[Andrew McDermott]

I wanted to explicitly target Ninja as my CMake build generator.

For example:

name: rtags
version: '0.1'
summary: rtags - a C/C++ source code indexer
description: |
  A C/C++ client/server indexer for c/c++/objc[++] with integration
  for Emacs based on clang.

parts:
  rtags:
    plugin: cmake
    source: https://github.com/Andersbakken/rtags.git
    configflags: [-GNinja]

But this always fails as snapcraft assumes CMake == "UNIX Makefiles" and always invokes 'make' irrespective of the specified CMake build generator.

Example output from `snapcraft build`:

-- Configuring done
-- Generating done
-- Build files have been written to: /home/aim/repos/github/frobware/rtags-snap/parts/rtags/build
make -j8
make: *** No targets specified and no makefile found. Stop.
Command '['/bin/sh', '/tmp/tmpsmpeb2_6', 'make', '-j8']' returned non-zero exit status 2

Whereas CMake correctly generated build.ninja

$ ls -l /home/aim/repos/github/frobware/rtags-snap/parts/rtags/build/build.ninja
-rw-rw-r-- 1 aim aim 118553 Feb 27 08:48 /home/aim/repos/github/frobware/rtags-snap/parts/rtags/build/build.ninja

[Cris Dywan (kalikiana)]

Two things need to happen to make this work: Snapcraft needs to read the generator specified with -G. In the case of ninja, it should install ninja-build (add it to self.build_packages). Bonus points for an error if an unsupported generator is being specified.

Maybe a dedicated options like "generator" is even clearer, where it would add -G accordingly, since it could use an enum with specific values.