There are three errors here actually.  The first one is a typo

:       AC_CHECK_DECL([SHA_DIGEST_LENGTH],
                       ^^^
should be SHA1_DIGEST_LENGTH
:                    [AC_DEFINE(HAVE_SHA1_AS_SHA1)],
:                    [AC_DEFINE(NEEDS_SHA1_DEFS)],
:                    [#include <sha1.h>])

the second is a missing header.  At least on OpenBSD sha1.h needs
sys/types.h too.

:       fi

With these fixed, the build still fails because of a re-definition of
SHA1_* constant.  It is because the (failed) test for sha.h sets
NEEDS_SHA1_DEFS.

This is my attempt at simplifying the checks.  We only look at sha.h
if the sha1.h check failed, and only if both fails it set
NEEDS_SHA1_DEFS.

diff /home/op/w/got-portable
commit - 415a43bb0c7491db8a907b4e5bbdf9894602d651
path + /home/op/w/got-portable
blob - 613855f12d450e8083c23a83e2880abb4c863cc3
file + configure.ac
--- configure.ac
+++ configure.ac
@@ -88,25 +88,20 @@ if test "x$ac_cv_header_sha_h" = xyes; then
 AM_CONDITIONAL([HAVE_SHA2], [test "x$ac_cv_header_sha2_h" = xyes])
 
 # Check for SHA1_DIGEST_LENGTH
-if test "x$ac_cv_header_sha_h" = xyes; then
-AC_CHECK_DECL([SHA_DIGEST_LENGTH],
-	      [AC_DEFINE(HAVE_SHA_AS_SHA1)],
-	      [AC_DEFINE(NEEDS_SHA1_DEFS)],
-	      [#include <sha.h>])
-fi
+AC_CHECK_DECL([SHA1_DIGEST_LENGTH],
+	[AC_DEFINE(HAVE_SHA1_AS_SHA1)],
+	[
+		dnl maybe sha.h is available
+		AC_CHECK_DECL([SHA_DIGETS_LENGTH],
+			[AC_DEFINE(HAVE_SHA_AS_SHA1)],
+			[AC_DEFINE(NEEDS_SHA1_DEFS)],
+			[#include <sha.h>])
+	],
+	[
+	#include <sys/types.h>
+	#include <sha1.h>
+	])
 
-if test "x$ac_cv_header_sha1_h" = xyes; then
-AC_CHECK_DECL([SHA_DIGEST_LENGTH],
-	      [AC_DEFINE(HAVE_SHA1_AS_SHA1)],
-	      [AC_DEFINE(NEEDS_SHA1_DEFS)],
-	      [#include <sha1.h>])
-fi
-
-if test "x$ac_cv_header_sha_h" = xno || test "x$ac_cv_header_sha1_h" = xno
-then
-	AC_DEFINE(NEEDS_SHA1_DEFS)
-fi
-
 # Checks for typ edefs, structures, and compiler characteristics.
 AC_CHECK_HEADER_STDBOOL
 AC_C_INLINE