[release] Fix pthread*test.

Add missing underscore in PTHREAD_CFLAGS.
Add missing returns in solver functions.
(cherry picked from commit 652f79bb4f424c8d9fe13b8ae481a41295a1eb49)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>
[documentation,release] Make the UMFPack class appear in the doxygen even when not configured with UMFPack

The user should see from the documentation that he/she could use UMFPack,
provided the missing external dependency is installed.
(cherry picked from commit a2c520aef3974cbed60ac7e024f9a07068b528e7)

Conflicts:
	dune/istl/umfpack.hh
[bugfix,documentation] Fix some spelling errors
(cherry picked from commit d37d0de2781f9a8b4dcd7b5eaf6e20bf79fcf174)

Conflicts:
	dune/istl/umfpack.hh