build: fix OpenSSL EC detection on macOS

build-aux/m4/bitcoin_secp.m4

@@ -38,6 +38,8 @@ AC_DEFUN([SECP_OPENSSL_CHECK],[
   fi
 if test x"$has_libcrypto" = x"yes" && test x"$has_openssl_ec" = x; then
   AC_MSG_CHECKING(for EC functions in libcrypto)
+  CPPFLAGS_TEMP="$CPPFLAGS"
+  CPPFLAGS="$CRYPTO_CPPFLAGS $CPPFLAGS"
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
     #include <openssl/ec.h>
     #include <openssl/ecdsa.h>
@@ -51,6 +53,7 @@ if test x"$has_libcrypto" = x"yes" && test x"$has_openssl_ec" = x; then
     ECDSA_SIG_free(sig_openssl);
   ]])],[has_openssl_ec=yes],[has_openssl_ec=no])
   AC_MSG_RESULT([$has_openssl_ec])
+  CPPFLAGS="$CPPFLAGS_TEMP"
 fi
 ])