Use __gcov_dump declaration from <gcov.h> if available

The function declaration provided by <gcov.h> is fine,
use it when <gcov.h> is available.

* configure.ac (AC_CHECK_HEADERS): Add gcov.h.
* defs.h [ENABLE_COVERAGE_GCOV && HAVE_GCOV_H]: Include <gcov.h>.

configure.ac

@@ -389,6 +389,7 @@ AC_CHECK_HEADERS(m4_normalize([
 	asm/guarded_storage.h
 	asm/sysmips.h
 	elf.h
+	gcov.h
 	iconv.h
 	inttypes.h
 	linux/bsg.h

defs.h

@@ -1469,7 +1469,11 @@ extern const struct_sysent *const sysent_vec[SUPPORTED_PERSONALITIES];
 extern struct inject_opts *inject_vec[SUPPORTED_PERSONALITIES];
 
 # ifdef ENABLE_COVERAGE_GCOV
+#  ifdef HAVE_GCOV_H
+#   include <gcov.h>
+#  else
 extern void __gcov_dump(void);
+#  endif
 #  define GCOV_DUMP __gcov_dump()
 # else
 #  define GCOV_DUMP