getpass: Check for nonnull prompt argument while avoiding warnings.
The prompt parameter to getpass() is declared as nonnull (using a GCC
nonnull attribute). Gnulib contains two implementations of this function,
one for POSIX, one for Windows. The Windows implementation checked for
a nonnull prompt, which caused a GCC warning. This commit fixes that by
avoiding the nonnull attribute when building getpass.c. The POSIX
implementation did not check for a nonnull prompt. This commit increases
the robustness by adding such a check.
* lib/getpass.c (_GL_ARG_NONNULL): Define to empty.
(getpass) [!_WIN32]: Print prompt only if nonnull.