Merge bitcoin-core/secp256k1#1273: build: Make SECP_VALGRIND_CHECK preserve CPPFLAGS

1ecb94e build: Make `SECP_VALGRIND_CHECK` preserve `CPPFLAGS` (Hennadii Stepanov)

Pull request description:

  It was overlooked in bitcoin#862 and bitcoin#1027.

ACKs for top commit:
  real-or-random:
    utACK 1ecb94e

Tree-SHA512: 263fc600ce9743e4aad767150f706bf7d4325dabb9c363ed57f08fe38faea94d7d1999804947cffeacbe698bb6d959ee6de3f6e50400050a390ecc0db957e426

Author: real-or-random

build-aux/m4/bitcoin_secp.m4

@@ -21,6 +21,7 @@ if test x"$has_valgrind" != x"yes"; then
     #  error "Valgrind does not support this platform."
     #endif
   ]])], [has_valgrind=yes])
+  CPPFLAGS="$CPPFLAGS_TEMP"
 fi
 AC_MSG_RESULT($has_valgrind)
 ])