gettext.h: add parentheses around N_ expansion if supported

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>