build: Make Valgrind include directory a system one

This change prevents warnings emitted for the Valgrind code.

Author: hebasto

CMakeLists.txt

@@ -117,7 +117,7 @@ if(SECP256K1_VALGRIND)
   find_package(Valgrind MODULE)
   if(Valgrind_FOUND)
     set(SECP256K1_VALGRIND ON)
-    include_directories(${Valgrind_INCLUDE_DIR})
+    include_directories(SYSTEM ${Valgrind_INCLUDE_DIR})
     add_definitions(-DVALGRIND)
   elseif(SECP256K1_VALGRIND STREQUAL "AUTO")
     set(SECP256K1_VALGRIND OFF)

configure.ac

@@ -57,7 +57,7 @@ case $host_os in
          # Homebrew where each one is located, then adjust paths accordingly.
          if $BREW list --versions valgrind >/dev/null; then
            valgrind_prefix=$($BREW --prefix valgrind 2>/dev/null)
-           VALGRIND_CPPFLAGS="-I$valgrind_prefix/include"
+           VALGRIND_CPPFLAGS="-nostdinc -isystem $valgrind_prefix/include"
          fi
        else
          AC_CHECK_PROG([PORT], port, port)