it's possible to build llvm with ninja by applying the following patch to the SlackBuild (inspired from BLFS):
---------------------
--- llvm.SlackBuild.old 2018-09-21 02:09:28.000000000 +0200
+++ llvm.SlackBuild 2018-12-16 20:37:30.539003072 +0100
@@ -145,40 +145,15 @@
-DLLVM_USE_OPROFILE=ON \
-DLLVM_BINUTILS_INCDIR=/usr/include \
-DCLANG_RESOURCE_DIR="../lib${LIBDIRSUFFIX}/clang/${VERSION}" \
- .. || exit 1
+ -Wno-dev -G Ninja .. &&
+ ninja || exit 1

# Breaks with one of the patches above. Maybe revisit later?
# -DBUILD_SHARED_LIBS=ON \

- # This seems to not like a parallel build, at least as of 7.0.0. I don't have
- # days to wait for the compile though, so let's just smack it with a hammer
- # fifty times before dropping back to a single-threaded build:
- for index in $(seq 1 50) ; do
- #make $NUMJOBS VERBOSE=1
make $NUMJOBS
- ERR_RESULT=$?
- if [ $ERR_RESULT = 0 ]; then
- break
- fi
- echo "*** PARALLEL MAKE RESTART NUMBER $index"
- done
- if [ ! $ERR_RESULT = 0 ]; then
- # Fifty more for the single thread:
- for index in $(seq 1 50) ; do
- #make VERBOSE=1
- make
- ERR_RESULT=$?
- if [ $ERR_RESULT = 0 ]; then
- break
- fi
- echo "*** NON-PARALLEL MAKE RESTART NUMBER $index"
- done
- fi
- if [ ! $ERR_RESULT = 0 ]; then
- exit 1
- fi

- make install DESTDIR=$PKG || exit 1
+ DESTDIR=$PKG ninja install || exit 1
cd ..

# Add symlinks for $ARCH-slackware-linux-{clang,clang++}:
-------------------------
parallel building doesn't seem to be a problem in that case

please wrap them in CODE tag

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Works here, just built 7.0.1 in parallel with no hammer.