src: Add sign-compare warnings to clang

src: Add sign-compare warnings to clang

For a while, GCC has generated warnings about sign errors. A common
mistake if compiling with clang was to accidentally introduce signedness
errors, which were picked up by the GCC builds.

This occurs due to an inconsistency in -Wall implementation between clang
and gcc: gcc includes sign-compare, clang does not.

See:
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wall
vs
https://clang.llvm.org/docs/DiagnosticsReference.html#wall

Note that sign-compare is included under -Wextra for clang:
https://clang.llvm.org/docs/DiagnosticsReference.html#wextra

Clang will now generate similar warnings with -Wsign-compare:
https://clang.llvm.org/docs/DiagnosticsReference.html#wsign-compare

Interestingly, if specified on its own, -Wsign-compare will include
C, whereas gcc -Wall affects C++ only. Therefore we must work around
this in the make file to emulate the GCC behaviour in clang builds.

Also fix a couple of warnings found in some tests.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>