[Buried headers] Change for the CMake-related files

[da115115]

Insulated the changes, derived from burying the headers, around CMake-related files only.

[da115115]

That pull request has been replaced by two other pull requests allowing to insulate specifically the changes respectively related to the source code and to the CMake-related framework.
The two new pull requests are:

• [Buried headers] Change for the source code #4 for the source code
• [Buried headers] Change for the CMake-related files #5 for the CMake-related framework

Note, though, that the pull requests are made against the master branch. Therefore, all the changes related to the directory structure change appear, but are not relevant here. I you want to see only the changes on the source code and/or the CMake-related framework, you can do the following on your local clone:
#==========================
git clone git://github.com/denisarnaud/soci.git socigit

cd socigit
git checkout master
git checkout src
git checkout src_build
git checkout src_buried_headers
git checkout src_build_buried_headers

To see the differences pertaining to the source code

git checkout src
git diff src_buried_headers ^src

To see the differences pertaining to the CMake-related framework

git checkout src_build
git diff src_build_buried_headers ^src
#==========================
In order to apply the changes:
git checkout src
git merge src_buried_headers
git checkout src_build
git merge src_build_buried_headers

git checkout master
git merge -s subtree --squash --no-commit src
git merge -s subtree --squash --no-commit src_build

git stat
git diff --cached

If there are some issues

git reset HEAD *
git checkout -- *