CMake Portgroup: activate CMAKE_EXPORT_COMPILE_COMMANDS

[RJVB (René Bertin)]

This is an enhancement suggestion complementary to the one filed under ticked #50246 .

CMake has the option to generate an additional file in the build directory, that contains the compile commands.This file is of little interest for building a port (it is comparable to the .CC_OPTIONS file) but its creation comes at negligible cost. It is however of considerable interest when the build directory is used with certain IDEs : KDevelop5 uses it to obtain the project contents. When absent, it will invoke a default cmake command with just enough arguments to interfere with the proper settings determined by the port.

My suggestion is thus to add -DCMAKE_EXPORT_COMPILE_COMMANDS=on to the list of cmake arguments, and to add a touch ${build.dir}/compile_commands.json to the post-configure block that also creates the cmake invocation file.

[RJVB (René Bertin)]

Turns out an empty compile_commands.json does generate errors in KDevelop; it should contain an empty json array instead.

[mf2k (Frank Schima)]

We have moved on to the cmake 1.1 portgroup. Please open a PR against that if any of these changes should go there.