reroute musl unistd.h weirdness

Musl is (of course) not glibc.  Its confstr(3) does not understand
_CS_GNU_LIBC_VERSION.  That's fair.  Problem is, its unistd.h has that
constant defined for unknown reason.  We cannot blindly say the libc is
glibc by looking at it.  Instead we have to kick it, then see if it
quacks like a duck.

See https://git.musl-libc.org/cgit/musl/tree/include/unistd.h

test/ruby/test_m17n_comb.rb

@@ -751,8 +751,14 @@ class TestM17NComb < Test::Unit::TestCase
   # glibc 2.16 or later denies salt contained other than [0-9A-Za-z./] #7312
   # we use this check to test strict and non-strict behavior separately #11045
   strict_crypt = if defined? Etc::CS_GNU_LIBC_VERSION
-    glibcver = Etc.confstr(Etc::CS_GNU_LIBC_VERSION).scan(/\d+/).map(&:to_i)
-    (glibcver <=> [2, 16]) >= 0
+    begin
+      confstr = Etc.confstr(Etc::CS_GNU_LIBC_VERSION)
+    rescue Errno::EINVAL
+      false
+    else
+      glibcver = confstr.scan(/\d+/).map(&:to_i)
+      (glibcver <=> [2, 16]) >= 0
+    end
   end
 
   def test_str_crypt