Make crypt and crypt_gensalt use thread-local output buffers.

This change makes crypt and crypt_gensalt as thread-safe as
they can be without changing their interfaces.  Solaris
already made this change, and it’s being discussed by glibc
(with suggestion that it should be pushed upstream to the C
and POSIX standards committees):
https://sourceware.org/ml/libc-alpha/2018-10/msg00437.html

Portable programs should still use the r-variants, though,
because this is not a guaranteed feature that is portable
to other implementations of these functions. Also it doesn’t
help threads to not clobber their corresponding output buffer
on a second call from within the same thread.

LICENSING

@@ -74,9 +74,10 @@ source tree.  For specific licensing terms consult the files themselves.
  * Public domain, written by Zack Weinberg et al.:
    byteorder.h, randombytes.c, test-byteorder.c
    test-alg-pbkdf-hmac-sha256.c
-   test-badsetting.c, test-crypt-badargs.c, test-getrandom-fallbacks.c,
-   test-getrandom-interface.c, test-symbols-compat.sh,
-   test-symbols-renames.sh, test-symbols-static.sh,
+   test-badsetting.c, test-crypt-badargs.c, test-crypt-multithread.c,
+   test-getrandom-fallbacks.c, test-getrandom-interface.c,
+   test-symbols-compat.sh, test-symbols-renames.sh,
+   test-symbols-static.sh,
    build-aux/scripts/gen-crypt-h,
    build-aux/scripts/gen-crypt-symbol-vers-h,
    build-aux/scripts/gen-libcrypt-map,
@@ -88,6 +89,14 @@ source tree.  For specific licensing terms consult the files themselves.
    GPL (v3 or later), with Autoconf exception:
    build-aux/m4/zw_automodern.m4, build-aux/m4/zw_simple_warnings.m4
 
+*  Copyright Steven G. Johnson, Daniel Richard G., Marc Stevens;
+   GPL (v3 or later), with Autoconf exception:
+   build-aux/m4/ax_pthread.m4
+
+*  Copyright Alan Woodland, Diego Elio Petteno;
+   GPL (v3 or later), with Autoconf exception:
+   build-aux/m4/ax_tls.m4
+
  * Copyright <vt at altlinux.org>; 0-clause BSD:
    crypt-yescrypt.c, test-crypt-yescrypt.c
 

Makefile.am

@@ -375,6 +375,7 @@ check_PROGRAMS = \
 	test/compile-strong-alias \
 	test/crypt-badargs \
 	test/crypt-gost-yescrypt \
+	test/crypt-multithread \
 	test/explicit-bzero \
 	test/gensalt \
 	test/gensalt-extradata \
@@ -478,6 +479,10 @@ test/symbols-compat.log test/symbols-compat.trs: test/TestCommon.pm
 test/symbols-renames.log test/symbols-renames.trs: test/TestCommon.pm
 test/symbols-static.log test/symbols-static.trs: test/TestCommon.pm
 
+# test/crypt-multithread.c uses pthreads.
+test_crypt_multithread_CFLAGS = $(PTHREAD_CFLAGS)
+test_crypt_multithread_LIBS = $(PTHREAD_LIBS)
+
 COMMON_TEST_OBJECTS = libcrypt.la
 
 test_badsalt_LDADD = $(COMMON_TEST_OBJECTS)
@@ -488,6 +493,7 @@ test_checksalt_LDADD = $(COMMON_TEST_OBJECTS)
 test_des_obsolete_LDADD = $(COMMON_TEST_OBJECTS)
 test_des_obsolete_r_LDADD = $(COMMON_TEST_OBJECTS)
 test_crypt_badargs_LDADD = $(COMMON_TEST_OBJECTS)
+test_crypt_multithread_LDADD = $(COMMON_TEST_OBJECTS)
 test_short_outbuf_LDADD = $(COMMON_TEST_OBJECTS)
 test_preferred_method_LDADD = $(COMMON_TEST_OBJECTS)
 test_special_char_salt_LDADD = $(COMMON_TEST_OBJECTS)