games/0ad/base: recognise MAKE_JOBS, don't build tests

Official build tags mention passing -jN to the update-workspaces.sh script,
which does make it use more jobs on my box, so do that.

Their build system is a very weird mix and I still see -j2 in the process list
with and without this diff, so something else is at work, but this diff does
not make that worse.

Compiling .cpp tests takes noticable time and are logged, but we never run
them, so disable for now.

Document -std=gnu++17 as seen in compile logs on amd64.

Feedback? Objection? OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/games/0ad/base/Makefile,v
retrieving revision 1.40
diff -u -p -r1.40 Makefile
--- Makefile 6 Mar 2023 13:37:15 -0000 1.40
+++ Makefile 11 Apr 2023 10:48:44 -0000
@@ -35,6 +35,7 @@ RUN_DEPENDS = devel/desktop-file-utils
games/0ad/data=${V}
MODULES = lang/python

+# gnu++17
COMPILER = base-clang ports-gcc

MODPY_RUNDEP = No
@@ -58,7 +59,8 @@ CONFIGURE_ENV += SHELL=sh

# XXX don't use --with-native-mozjs because 0ad needs to stay in sync
# with it: updates to mozjs could break the multiplayer.
-CONFIGURE_ARGS += --without-nvtt \
+CONFIGURE_ARGS += -j${MAKE_JOBS} \
+ --without-nvtt \
--datadir=${PREFIX}/share/0ad \
--libdir=${PREFIX}/lib \
--minimal-flags
@@ -97,6 +99,11 @@ do-install:

# most tests fail without nvtt
NO_TEST = Yes
+# avoid building unused tests in ${WRKSRC}/Makefile
+MAKE_FLAGS += cxxtestroot_config='' \
+ mocks_test_config='' \
+ test_config=''
+
#do-test:
# cd ${WRKDIST}/binaries/system && ./test