fix make -j passing build cores

Makefile

@@ -1,10 +1,7 @@
-# Parse -j from MAKEFLAGS for parallelism.
-GERBIL_BUILD_CORES := $(or $(patsubst -j%,%,$(filter -j%,$(MAKEFLAGS))),1) 
-
 all: build
 
 build:
-	GERBIL_BUILD_CORES=$(GERBIL_BUILD_CORES) ./build.sh
+	GERBIL_BUILD_FLAGS="$(MAKEFLAGS)" ./build.sh
 
 install:
 	./install.sh

doc/guide/README.md

@@ -46,9 +46,13 @@ cd into the `gerbil` directory.
 Gerbil takes quite a while to compile, if you wish it to build faster
 by utilizing multiple cores, you can:
 ```
-make -j <number-of-cores>
+make -j<number-of-cores>
 ```
 
+Alternatively, you can set the `GERBIL_BUILD_CORES` environment
+variable to the number of cores you want to use.
+
+
 If you are using the default configuration, you can build Gerbil simply with:
 ```bash
 $ ./configure && make && sudo make install

docker/Dockerfile

@@ -51,7 +51,7 @@ RUN cd /opt && git clone https://github.com/vyzo/gerbil gerbil-src
 RUN cd /opt/gerbil-src && git fetch -a && git fetch --tags && git checkout ${gerbil_version} \
     && eval ./configure --prefix=/opt/gerbil --enable-shared=no ${configure_args}
 
-RUN cd /opt/gerbil-src && make -j ${GERBIL_BUILD_CORES} && make install
+RUN cd /opt/gerbil-src && make && make install
 
 FROM gerbil as final
 RUN rm -rf /opt/gerbil-src /src/leveldb /src/lmdb

src/build.sh

@@ -6,6 +6,20 @@ cd $(dirname "$0") # Change to this directory
 # Assuming this script is run with: `cd $GERBIL_BASE/src && ./build.sh`
 #===============================================================================
 
+# utility to get number of cores from make -j
+build_flags_cores() {
+    for x in $1; do
+        case $x in
+            -j*)
+                echo ${x##-j}
+                break
+                ;;
+            *)
+                ;;
+        esac
+    done
+}
+
 # Check for GERBIL_PREFIX being set
 # This is necessary for the bach build script to set the correct RUNPATH in the
 # gerbil binary.
@@ -42,6 +56,18 @@ else
 fi
 export LD_LIBRARY_PATH
 
+if [ "x${GERBIL_BUILD_FLAGS:-}" != "x" ]; then
+    num_cores=$(build_flags_cores "${GERBIL_BUILD_FLAGS}")
+    if [ "x${num_cores:-}" != "x" ]; then
+        GERBIL_BUILD_CORES=${num_cores}
+        export GERBIL_BUILD_CORES
+    fi
+fi
+
+if [ "x${GERBIL_BUILD_CORES:-}" != "x" ]; then
+    echo "using ${GERBIL_BUILD_CORES} cores for the build"
+fi
+
 #===============================================================================
 ## feedback
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