ci: use two parallel build jobs on Travis-CI

Change-Id: I0d37709bd81bbff890c932b77dc5be696ac35569
diff --git a/.jenkins.d/10-build.sh b/.jenkins.d/10-build.sh
index 6c4928d..6844e93 100755
--- a/.jenkins.d/10-build.sh
+++ b/.jenkins.d/10-build.sh
@@ -12,22 +12,22 @@
 sudo rm -f /usr/local/lib/pkgconfig/libndn-cxx.pc
 
 # Cleanup
-sudo env "PATH=$PATH" ./waf -j1 --color=yes distclean
+sudo env "PATH=$PATH" ./waf --color=yes distclean
 
 if [[ $JOB_NAME != *"code-coverage" && $JOB_NAME != *"limited-build" ]]; then
   # Configure/build static library in optimized mode with tests
-  ./waf -j1 --color=yes configure --enable-static --disable-shared --with-tests
-  ./waf -j1 --color=yes build
+  ./waf --color=yes configure --enable-static --disable-shared --with-tests
+  ./waf --color=yes build -j${WAF_JOBS:-1}
 
   # Cleanup
-  sudo env "PATH=$PATH" ./waf -j1 --color=yes distclean
+  sudo env "PATH=$PATH" ./waf --color=yes distclean
 
   # Configure/build static and shared library in optimized mode without tests
-  ./waf -j1 --color=yes configure --enable-static --enable-shared
-  ./waf -j1 --color=yes build
+  ./waf --color=yes configure --enable-static --enable-shared
+  ./waf --color=yes build -j${WAF_JOBS:-1}
 
   # Cleanup
-  sudo env "PATH=$PATH" ./waf -j1 --color=yes distclean
+  sudo env "PATH=$PATH" ./waf --color=yes distclean
 fi
 
 # Configure/build shared library in debug mode with tests/examples and without precompiled headers
@@ -36,13 +36,13 @@
 elif [[ -n $BUILD_WITH_ASAN || -z $TRAVIS ]]; then
     ASAN="--with-sanitizer=address"
 fi
-./waf -j1 --color=yes configure --disable-static --enable-shared --debug --with-tests --with-examples --without-pch $COVERAGE $ASAN
-./waf -j1 --color=yes build
+./waf --color=yes configure --disable-static --enable-shared --debug --with-tests --with-examples --without-pch $COVERAGE $ASAN
+./waf --color=yes build -j${WAF_JOBS:-1}
 
 # (tests will be run against debug version)
 
 # Install
-sudo env "PATH=$PATH" ./waf -j1 --color=yes install
+sudo env "PATH=$PATH" ./waf --color=yes install
 
 if has Linux $NODE_LABELS; then
     sudo ldconfig
diff --git a/.jenkins.d/README.md b/.jenkins.d/README.md
index b1bd93a..f6e3167 100644
--- a/.jenkins.d/README.md
+++ b/.jenkins.d/README.md
@@ -29,3 +29,5 @@
   * empty: default build process
   * `code-coverage` (Linux OS is assumed): debug build with tests and code coverage analysis
   * `limited-build`: only a single debug build with tests
+
+- `WAF_JOBS`: number of parallel build jobs used by waf, defaults to 1.