Makefile: Solaris needs HAVE_ALLOCA_H for alloca()

There is special handling in compat/regex/regex.c for the GNU compiler
to define alloca to __builtin_alloca, but the native compiler must include
alloca.h which happens when HAVE_ALLOCA_H is defined.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -753,7 +753,7 @@ ifeq ($(uname_S),SunOS)
 	endif
 	INSTALL = /usr/ucb/install
 	TAR = gtar
-	BASIC_CFLAGS += -D__EXTENSIONS__ -D__sun__
+	BASIC_CFLAGS += -D__EXTENSIONS__ -D__sun__ -DHAVE_ALLOCA_H
 endif
 ifeq ($(uname_O),Cygwin)
 	NO_D_TYPE_IN_DIRENT = YesPlease