Ask brew for valgrind include path

Valgrind is typically installed using brew on macOS. This commit
makes ./configure detect this case set the appropriate include
directory (in the same way as we already do for openssl and gmp).

build-aux/m4/bitcoin_secp.m4

@@ -87,3 +87,11 @@ if test x"$has_gmp" != x"yes"; then
   LIBS="$LIBS_TEMP"
 fi
 ])
+
+AC_DEFUN([SECP_VALGRIND_CHECK],[
+if test x"$has_valgrind" != x"yes"; then
+  CPPFLAGS_TEMP="$CPPFLAGS"
+  CPPFLAGS="$VALGRIND_CPPFLAGS $CPPFLAGS"
+  AC_CHECK_HEADER([valgrind/memcheck.h], [has_valgrind=yes; AC_DEFINE(HAVE_VALGRIND,1,[Define this symbol if valgrind is installed])])
+fi
+])