Check if OpenMP is available before using it

configure.ac

@@ -689,6 +689,15 @@ if test x$enable_rust != xno; then
   RUST_LIBS="-lrustzcash"
 fi
 
+dnl Check for OpenMP support
+AX_OPENMP(
+  [AC_DEFINE(HAVE_OPENMP, 1, [Define if OpenMP is enabled])
+   AM_CONDITIONAL([HAVE_OPENMP], [true])
+   CXXFLAGS="$CXXFLAGS $OPENMP_CXXFLAGS"],
+  [AC_MSG_WARN([OpenMP not supported, disabling multithreading])
+   AC_DEFINE(HAVE_OPENMP, 0, [Define if OpenMP is enabled])
+   AM_CONDITIONAL([HAVE_OPENMP], [false])])
+
 # Gitian uses a config.site that sets depends_prefix, and then sets --prefix=/
 # build.sh just uses --prefix
 if test x$depends_prefix != x; then
@@ -697,7 +706,7 @@ else
   LIBSNARK_DEPINST="$prefix"
 fi
 
-LIBZCASH_LIBS="-lgmp -lgmpxx -lboost_system-mt -lcrypto -lsodium -fopenmp $RUST_LIBS"
+LIBZCASH_LIBS="-lgmp -lgmpxx -lboost_system-mt -lcrypto -lsodium $RUST_LIBS"
 
 CXXFLAGS_TEMP="$CXXFLAGS"
 LIBS_TEMP="$LIBS"