hash.h: move SHA-1 implementation selection into a header file

Many developers use functionality in their editors that allows for quick
syntax checks, including warning about questionable constructs.  This
functionality allows rapid development with fewer errors.  However, such
functionality generally does not allow the specification of
project-specific defines or command-line options.

Since the SHA1_HEADER include is not defined in such a case,
developers see spurious errors when using these tools.  Furthermore,
there are known implementations of "cc" whose '#include' is unhappy
with this construct.

Instead of using SHA1_HEADER, create a hash.h header and use #if
and #elif to select the desired header.  Have the Makefile pass an
appropriate option to help the header select the right implementation to
use.

[jc: make BLK_SHA1 the fallback default as discussed on list,
e.g. <20170314201424.vccij5z2ortq4a4o@sigill.intra.peff.net>; also
remove SHA1_HEADER and SHA1_HEADER_SQ that are no longer used].

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1387,19 +1387,19 @@ ifdef APPLE_COMMON_CRYPTO
 endif
 
 ifdef BLK_SHA1
-	SHA1_HEADER = "block-sha1/sha1.h"
 	LIB_OBJS += block-sha1/sha1.o
+	BASIC_CFLAGS += -DSHA1_BLK
 else
 ifdef PPC_SHA1
-	SHA1_HEADER = "ppc/sha1.h"
 	LIB_OBJS += ppc/sha1.o ppc/sha1ppc.o
+	BASIC_CFLAGS += -DSHA1_PPC
 else
 ifdef APPLE_COMMON_CRYPTO
 	COMPAT_CFLAGS += -DCOMMON_DIGEST_FOR_OPENSSL
-	SHA1_HEADER = <CommonCrypto/CommonDigest.h>
+	BASIC_CFLAGS += -DSHA1_APPLE
 else
-	SHA1_HEADER = <openssl/sha.h>
 	EXTLIBS += $(LIB_4_CRYPTO)
+	BASIC_CFLAGS += -DSHA1_OPENSSL
 endif
 endif
 endif
@@ -1591,7 +1591,6 @@ endif
 
 # Shell quote (do not use $(call) to accommodate ancient setups);
 
-SHA1_HEADER_SQ = $(subst ','\'',$(SHA1_HEADER))
 ETC_GITCONFIG_SQ = $(subst ','\'',$(ETC_GITCONFIG))
 ETC_GITATTRIBUTES_SQ = $(subst ','\'',$(ETC_GITATTRIBUTES))
 
@@ -1624,8 +1623,7 @@ PERLLIB_EXTRA_SQ = $(subst ','\'',$(PERLLIB_EXTRA))
 # from the dependency list, that would make each entry appear twice.
 LIBS = $(filter-out %.o, $(GITLIBS)) $(EXTLIBS)
 
-BASIC_CFLAGS += -DSHA1_HEADER='$(SHA1_HEADER_SQ)' \
-	$(COMPAT_CFLAGS)
+BASIC_CFLAGS += $(COMPAT_CFLAGS)
 LIB_OBJS += $(COMPAT_OBJS)
 
 # Quote for C

cache.h

@@ -10,8 +10,8 @@
 #include "trace.h"
 #include "string-list.h"
 #include "pack-revindex.h"
+#include "hash.h"
 
-#include SHA1_HEADER
 #ifndef platform_SHA_CTX
 /*
  * platform's underlying implementation of SHA-1; could be OpenSSL,

hash.h

@@ -0,0 +1,14 @@
+#ifndef HASH_H
+#define HASH_H
+
+#if defined(SHA1_PPC)
+#include "ppc/sha1.h"
+#elif defined(SHA1_APPLE)
+#include <CommonCrypto/CommonDigest.h>
+#elif defined(SHA1_OPENSSL)
+#include <openssl/sha.h>
+#else /* SHA1_BLK */
+#include "block-sha1/sha1.h"
+#endif
+
+#endif