liblzma: Fix a build failure related to external SHA-256 support.

If an appropriate header and structure were found by configure,
but a library with a usable SHA-256 functions wasn't, the build
failed.

src/liblzma/check/check.h

diff --git a/src/liblzma/check/check.h b/src/liblzma/check/check.h
index 0f96f65..b8902f0 100644
--- a/src/liblzma/check/check.h
+++ b/src/liblzma/check/check.h
@@ -15,7 +15,19 @@
 
 #include "common.h"
 
-#if defined(HAVE_COMMONCRYPTO_COMMONDIGEST_H)
+// If either the data type or the function for external SHA-256 is missing,
+// use the internal SHA-256 code.
+#if !(defined(HAVE_CC_SHA256_CTX) || defined(HAVE_SHA256_CTX) \
+			|| defined(HAVE_SHA2_CTX)) \
+		|| !(defined(HAVE_CC_SHA256_INIT) \
+			|| defined(HAVE_SHA256_INIT) \
+			|| defined(HAVE_SHA256INIT))
+#	define HAVE_INTERNAL_SHA256 1
+#endif
+
+#if defined(HAVE_INTERNAL_SHA256)
+// Nothing
+#elif defined(HAVE_COMMONCRYPTO_COMMONDIGEST_H)
 #	include <CommonCrypto/CommonDigest.h>
 #elif defined(HAVE_SHA256_H)
 #	include <sys/types.h>
@@ -28,13 +40,7 @@
 #	include <minix/sha2.h>
 #endif
 
-#if defined(HAVE_CC_SHA256_CTX)
-typedef CC_SHA256_CTX lzma_sha256_state;
-#elif defined(HAVE_SHA256_CTX)
-typedef SHA256_CTX lzma_sha256_state;
-#elif defined(HAVE_SHA2_CTX)
-typedef SHA2_CTX lzma_sha256_state;
-#else
+#if defined(HAVE_INTERNAL_SHA256)
 /// State for the internal SHA-256 implementation
 typedef struct {
 	/// Internal state
@@ -43,9 +49,17 @@
 	/// Size of the message excluding padding
 	uint64_t size;
 } lzma_sha256_state;
+#elif defined(HAVE_CC_SHA256_CTX)
+typedef CC_SHA256_CTX lzma_sha256_state;
+#elif defined(HAVE_SHA256_CTX)
+typedef SHA256_CTX lzma_sha256_state;
+#elif defined(HAVE_SHA2_CTX)
+typedef SHA2_CTX lzma_sha256_state;
 #endif
 
-#if defined(HAVE_CC_SHA256_INIT)
+#if defined(HAVE_INTERNAL_SHA256)
+// Nothing
+#elif defined(HAVE_CC_SHA256_INIT)
 #	define LZMA_SHA256FUNC(x) CC_SHA256_ ## x
 #elif defined(HAVE_SHA256_INIT)
 #	define LZMA_SHA256FUNC(x) SHA256_ ## x