Removes the ambiguity around what 'ninja' thing needs to be installed, since there are many tools named 'ninja'.

Building.md

@@ -1,7 +1,12 @@
 ### Building and testing with Meson/Ninja
 Thanks to David Seifert (@SoapGentoo), we (the maintainers) now use [meson](http://mesonbuild.com/) and [ninja](https://ninja-build.org/) to build for debugging, as well as for continuous integration (see [`meson_builder.sh`](../blob/master/.travis_scripts/meson_builder.sh) ). Other systems may work, but minor things like version strings might break.
 
-First, install both meson (which requires Python3) and ninja.
+First, install both meson (which requires Python3) and ninja (also Python3):
+
+    # installs meson
+    pip install meson
+    # installs ninja
+    pip install ninja
 
 Then,
 
@@ -13,6 +18,8 @@ Then,
     meson --buildtype ${BUILD_TYPE} --default-library ${LIB_TYPE} . build-${LIB_TYPE}
     ninja -v -C build-${LIB_TYPE} test
 
+
+
 ### Building and testing with CMake
 (Deprecated, but still works for now. The version string may soon be wrong.)