Add vasprintf() and asprintf() functions missing on AIX

These functions are used by code in the library, even though these
functions started as GNU extensions, they are present in all BSDs,
so we expose them as part of our interface on AIX.

configure.ac

@@ -44,6 +44,7 @@ is_windows=no
 abi_accmode=unknown
 abi_arc4random=unknown
 abi_arc4random_stir=unknown
+abi_asprintf=unknown
 abi_bsd_getopt=unknown
 abi_closefrom=unknown
 abi_err=unknown
@@ -89,6 +90,7 @@ AS_CASE([$host_os],
     #abi_arc4random=no
     abi_arc4random=yes
     abi_arc4random_stir=yes
+    abi_asprintf=no
     abi_bsd_getopt=yes
     # DROP: On glibc >= 2.34.
     #abi_closefrom=no
@@ -145,6 +147,7 @@ AS_CASE([$host_os],
     abi_accmode=yes
     abi_arc4random=yes
     abi_arc4random_stir=yes
+    abi_asprintf=no
     # DROP: On musl >= 0.9.7, optreset got implemented.
     #abi_bsd_getopt=no
     abi_bsd_getopt=yes
@@ -208,6 +211,7 @@ AS_CASE([$host_os],
     abi_accmode=no
     abi_arc4random=no
     abi_arc4random_stir=no
+    abi_asprintf=no
     abi_bsd_getopt=no
     abi_closefrom=yes
     abi_err=no
@@ -253,6 +257,7 @@ AS_CASE([$host_os],
     abi_accmode=yes
     abi_arc4random=no
     abi_arc4random_stir=no
+    abi_asprintf=no
     abi_bsd_getopt=yes
     abi_closefrom=no
     abi_err=no
@@ -465,6 +470,8 @@ LIBBSD_CHECK_PROGNAME
 LIBBSD_CHECK_REGISTER_ATFORK
 
 AC_CHECK_FUNCS([\
+  vasprintf \
+  asprintf \
   clearenv \
   dirfd \
   flock \
@@ -490,6 +497,7 @@ AM_CONDITIONAL([HAVE_GETENTROPY], [test "x$ac_cv_func_getentropy" = "xyes"])
 LIBBSD_SELECT_ABI([accmode], [setmode()/getmode()])
 LIBBSD_SELECT_ABI([arc4random], [arc4random() API])
 LIBBSD_SELECT_ABI([arc4random_stir], [arc4random_stir()/arc4random_addrandom()])
+LIBBSD_SELECT_ABI([asprintf], [vasprintf()/asprintf()])
 LIBBSD_SELECT_ABI([bsd_getopt], [BSD getopt()])
 LIBBSD_SELECT_ABI([closefrom], [closefrom()])
 LIBBSD_SELECT_ABI([err], [err API])