WCDH: Make it possible to generate multiple files.

Extend the write_compiler_detection_header interface to allow
specifying a location for supplementary files, and getting the
list of resulting files as a variable.
WCDH: Make the header -Wundef safe for the C language.

The __STDC_VERSION__ macro may be defined or not depending on the
implementation dialect of C.  Test that it is defined before testing
its value.

The CXX tests do not need such a change because they define __cplusplus
in all dialects.
ccmake: Add CMAKE_USE_SYSTEM_FORM option to use libform instead of cmForm
Remove note disallowing use of some string and vector API.

The compilers introducing these limitations are no longer supported
as host compilers.
return: Add test for returning inside a foreach loop

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>
Merge branch 'C-features-Wundef' into release
Merge topic 'C-features-Wundef'

d0af0fae WCDH: Make the header -Wundef safe for the C language.
Merge topic 'WCDH-multi-file'

4cf5179c WCDH: Make it possible to generate multiple files.
Merge topic 'ccmake-system-form'

6b045c3b ccmake: Add CMAKE_USE_SYSTEM_FORM option to use libform instead of cmForm
Merge topic 'remove-developer-forbidden-API'

beaea609 Remove note disallowing use of some string and vector API.
Merge topic 'return-test-foreach'

d124c411 return: Add test for returning inside a foreach loop
Merge branch 'release'