Add OBJECT_FILE_DIR rule placeholder for compilation lines

Some compilers do not offer an option to specify the path to the object
file, but rather only to the directory in which to place the object
file.  See issue 14876 for some examples.  Add a new OBJECT_FILE_DIR
placeholder to specify the directory containing the object file for the
current compilation.  This may differ from the main target OBJECT_DIR
when the object corresponds to a source in a subdirectory.
Xcode: Fix single-configuration generation for version 1.5

In commit 84fdc992 (stringapi: Pass configuration names as strings,
2014-02-09) a few code paths for the Xcode 1.5 single-configuration
generator were not updated to use an empty configuration name instead of
a NULL pointer when no configuration is specified in CMAKE_BUILD_TYPE.
Fix them now.
tests: allow RelWithDebInfo and MinSizeRel configs to work
FindOpenGL: Provide imported targets for GL and GLU

Create OpenGL::GL and OpenGL::GLU imported targets using the locations
found.
Help: Add notes about new OpenWatcom compiler id and versioning
Merge topic 'watcom-overhaul-notes'

559ae189 Help: Add notes about new OpenWatcom compiler id and versioning
Merge topic 'dev/fix-complex-tests'

7db77a82 tests: allow RelWithDebInfo and MinSizeRel configs to work
Merge topic 'compile-OBJECT_FILE_DIR'

8256ccb7 Add OBJECT_FILE_DIR rule placeholder for compilation lines
Merge topic 'xcode-15-string-apis'

23dc6aa1 Xcode: Fix single-configuration generation for version 1.5
Merge topic 'FindOpenGL-target'

b7804cb6 FindOpenGL: Provide imported targets for GL and GLU