build: Move ABI selection at the top of configure.ac

Merge the existing host_os block for the OS detection with the ABI
selection one, as these are related. This way we will be able to make
some of the latter checks conditional on the selected ABI.

configure.ac

@@ -34,12 +34,71 @@ AM_PROG_AR
 LT_INIT
 LIBBSD_LINKER_VERSION_SCRIPT
 
+## Select library ABI to expose.
+
 is_windows=no
+
+need_arc4random=yes
+need_bsd_getopt=yes
+need_err=yes
+need_errc=yes
+need_fpurge=yes
+need_funopen=yes
+need_md5=yes
+need_name_from_id=yes
+need_nlist=yes
+need_progname=yes
+need_strl=yes
+need_strmode=yes
+need_wcsl=yes
+
 AS_CASE([$host_os],
+  [*-gnu*], [
+    # On glibc >= 2.38, strlcpy() and strlcat() got added,
+    # so these could then be dropped on the next SOVERSION bump.
+    #need_strl=no
+    need_err=no
+  ],
+  [*-musl*], [
+    # On musl >= 0.9.7, optreset got implemented, so bsd_getopt() can then
+    # be dropped on the next SOVERSION bump.
+    #need_bsd_getopt=no
+    need_err=no
+    # On musl >= 1.1.19, fopencookie() got implemented, and because we were
+    # checking for its presence to decide whether to build funopen(), it got
+    # included in builds even when previously it had not been included, which
+    # is partially an ABI issue, but given that disabling it now would be
+    # worse, we'll ignore this as this is only a problem with downgrades. And
+    # enable it explicitly
+    need_funopen=yes
+    # On musl >= 0.5.0, strlcpy() and strlcat() were already implemented,
+    # so these can then be dropped on the next SOVERSION bump.
+    #need_strl=no
+  ],
+  [darwin*], [
+    # On macOS these are provided by the system, and libbsd has never built
+    # there, so we can avoid providing these with no ABI breakage.
+    need_arc4random=no
+    need_bsd_getopt=no
+    need_err=no
+    need_errc=no
+    need_fpurge=no
+    # On macOS we do not have fopencookie(), and cannot implement it.
+    need_funopen=no
+    need_md5=no
+    need_name_from_id=no
+    need_nlist=no
+    need_progname=no
+    need_strl=no
+    need_strmode=no
+    need_transparent_libmd=no
+    need_wcsl=no
+  ],
   [mingw*], [
     is_windows=yes
   ],
 )
+
 AM_CONDITIONAL([OS_WINDOWS], [test "x$is_windows" = "xyes"])
 
 # Checks for programs.
@@ -210,63 +269,6 @@ AC_CHECK_FUNCS([\
   group_from_gid \
 ])
 
-need_arc4random=yes
-need_bsd_getopt=yes
-need_err=yes
-need_errc=yes
-need_progname=yes
-need_md5=yes
-need_nlist=yes
-need_strl=yes
-need_wcsl=yes
-need_strmode=yes
-need_name_from_id=yes
-need_fpurge=yes
-need_funopen=yes
-AS_CASE([$host_os],
-  [*-gnu*], [
-    # On glibc >= 2.38, strlcpy() and strlcat() got added,
-    # so these could then be dropped on the next SOVERSION bump.
-    #need_strl=no
-    need_err=no
-  ],
-  [*-musl*], [
-    # On musl >= 0.5.0, strlcpy() and strlcat() were already implemented,
-    # so these can then be dropped on the next SOVERSION bump.
-    #need_strl=no
-    # On musl >= 0.9.7, optreset got implemented, so bsd_getopt() can then
-    # be dropped on the next SOVERSION bump.
-    #need_bsd_getopt=no
-    need_err=no
-    # On musl >= 1.1.19, fopencookie() got implemented, and because we were
-    # checking for its presence to decide whether to build funopen(), it got
-    # included in builds even when previously it had not been included, which
-    # is partially an ABI issue, but given that disabling it now would be
-    # worse, we'll ignore this as this is only a problem with downgrades. And
-    # enable it explicitly
-    need_funopen=yes
-  ],
-  [darwin*], [
-    # On macOS these are provided by the system, and libbsd has never built
-    # there, so we can avoid providing these with no ABI breakage.
-    need_arc4random=no
-    need_bsd_getopt=no
-    need_err=no
-    need_errc=no
-    need_progname=no
-    need_transparent_libmd=no
-    need_md5=no
-    need_nlist=no
-    need_strl=no
-    need_wcsl=no
-    need_strmode=no
-    need_name_from_id=no
-    need_fpurge=no
-    # On macOS we do not have fopencookie(), and cannot implement it.
-    need_funopen=no
-  ],
-)
-
 AM_CONDITIONAL([HAVE_GETENTROPY], [test "x$ac_cv_func_getentropy" = "xyes"])
 
 AM_CONDITIONAL([NEED_ARC4RANDOM], [test "x$need_arc4random" = "xyes"])