Not all C libraries are glibc, or impersonating it; for example, musl
does not pretend to be any version of glibc. Thus, building on musl
fails when openssl is disabled, because no random-providing function is
detected, although musl does provide getrandom().
uClibc-ng on the other hand, can impersonate glibc, but availability of
getrandom() is not guatranteed there: getrandom() can be compiled out of
uClinbc-ng, or uClibc-ng can be too old to have it.
Add a configure-time check that getrandom() is available, as a fallback
when TLS is not enabled (and thus openssl is not used), and when not on
Win32 (where getting random numbers is always possible, at least from a
build perspective).
However, building with the plain Makefiles should keep working, so
slightly rework the defines checks in the code to account for the fact
that HAVE_GETRANDOM may already be defined at configure time.
Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
