gettext.h: add parentheses around N_ expansion if supported

The gettext N_ macro is used to mark strings for translation without actually translating them. At runtime the string is expected to be passed to the gettext API for translation. If two N_ macro invocations appear next to each other with only whitespace (or nothing at all) between them, the two separate strings will be marked for translation, but the preprocessor will then silently combine the strings into one and at runtime the string passed to gettext will not match the strings that were translated so no translation will actually occur. Avoid this by adding parentheses around the expansion of the N_ macro so that instead of ending up with two adjacent strings that are then combined by the preprocessor, two adjacent strings surrounded by parentheses result instead which causes a compile error so the mistake can be quickly found and corrected. However, since these string literals are typically assigned to static variables and not all compilers support parenthesized string literal assignments, allow this to be controlled by the Makefile with the default only enabled when the compiler is known to support the syntax. For now only __GNUC__ enables this by default which covers both gcc and clang which should result in early detection of any adjacent N_ macros. Although the necessary tests make the affected files a bit less elegant, the benefit of avoiding propagation of a translation- marking error to all the translation teams thus creating extra work for them when the error is eventually detected and fixed would seem to outweigh the minor inelegance the additional configuration tests introduce. Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Kyle J. McKay <mackyle@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>