contrib/buildsystems: ignore invalidcontinue.obj

contrib/buildsystems: ignore invalidcontinue.obj

Since 4b623d8 (MSVC: link in invalidcontinue.obj for better POSIX
compatibility, 2014-03-29), invalidcontinue.obj is linked in the MSVC
build, but it was not parsed correctly by the buildsystem. Ignore it, as
it is known to Visual Studio and will be handled elsewhere.

Also only substitute filenames ending with .o when generating the
source .c filename, otherwise we would start to expect .cbj files to
generate .obj files (which are not generated by our build)...

In the future there may be source files that produce .obj files
so keep the two issues (.obj files with & without source files)
separate.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Duncan Smart <duncan.smart@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>