add instructions to install ccache

[haubourg]

Current instructions were working even if ccache was not installed.

Description

Include a few sentences describing the overall goals for this PR (pull request). If applicable also add screenshots.

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and containt sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[m-kuhn]

Thanks @haubourg
You will need to also adjust the t2t files in the doc folder (this file is generated)

[m-kuhn]

@haubourg bump

[haubourg]

@m-kuhn i'm still alive but moving home to a remote place surrounded by wolves and mountains :)

[haubourg]

@m-kuhn hey, it's a bit hard to find informations about current workflow.
txt2tags --encoding=utf8 INSTALL.t2t does the job for doc/INSTALL.html, but shall I also generate the INSTALL text file (the one I modified at start) so that we have a single source of truth?

[haubourg]

@m-kuhn Should be better now.

[m-kuhn]

Manually merged in 8dfa021

[haubourg]

ah, so it is possible that you push commits to my branch in my fork, didn't know that.
I was at two inches to rebase and fix a clean PR :)

[haubourg]

thanks !

[m-kuhn]

ah, so it is possible that you push commits to my branch in my fork, didn't know that.

Only on pull request branches, you can deactivate this when creating a new pull request.

[jef-n]

@haubourg there's the cmake target t2tdoc for this (updates INSTALL & INSTALL.html in the build directory)

[haubourg]

@jef-n Thanks for the pointer. And then I have to copy them back to the sources right?