c++: Differentiate scanning logic from C

c++: Differentiate scanning logic from C

To enable raw string literals only in C++, add a flag indicating
the current source language is C++.
Suggested by Vaclav Slavik at:
<https://savannah.gnu.org/bugs/?43970>.

* gettext-tools/src/x-c.h (extract_cxx): New declaration.
(SCANNERS_C): Use extract_cxx for "C++".
* gettext-tools/src/x-c.c (cxx_extensions): New variable.
(phase5_get): Respect cxx_extensions and recognize raw string
literals only when it is set.
(extract_cxx): New function.
(extract_c, extract_objc): Reset cxx_extensions.

* gettext-tools/tests/xgettext-c-20: Adjust the source file name
from *.c to *.cc to enable handling of raw string literals.