David Faure and Patrick Spendrin have convinced me that NAME_PREFIX
should be informational only, and not be used to prevent clashes, since
it makes things confusing when you run tests manually.

This is a SIC change (although in practice only kio and kconfig should
be affected, currently).

REVIEW: 118768

CCMAIL: kde-frameworks-devel@kde.org
CCMAIL: kde-buildsystem@kde.org

While the tests in ECM are not "built" as such (at least, not until they
are run), disabling the tests might be desirable to avoid the compiler
checks and to make the whole build process architecture-independent.

REVIEW: 118498

Setting the language for ECM's project() call to C had unanticipated
side-effects - notably that the installed version file required the
architecture to match the one used at build time.

Instead, we make the tests a sub-project, setting up C as the language
there (since most of the tests do use C, albeit slightly indirectly).

REVIEW: 118498

Move the detailed testing of KDEInstallDirs from ExecuteKDEModules to a
subdir of KDEInstallDirsTest. This is where you would expect to find it,
and it also makes sure that other KDE modules are not affecting the
test.

This also makes the KDEInstallDirs/not_cache_variable regression test
work the same way as the other tests, doing a double-configure and
build. While not stricly necessary to catch the original issue, it does
ensure that the problem does not appear when reconfiguring either.

Although ECM does not make use of a compiler directly, the language
affects the search path for CMake packages; in particular, a package
installed to /usr/lib64/cmake will not be found if NONE is passed as the
language argument to project(). This meant that a 64-bit version of
Qt5LinguistTools would not be found on systems where 64-bit libraries
are not installed in the "default architecture" location (/usr/lib).

With this change, the configure step performs some otherwise-unnecessary
tests. We minimise this by explicitly specifying the C language (which
is also what some of the tests use), rather than letting it be the
default (which is C and C++).

REVIEW: 118374