Add automatic way to get GSL CFLAGS and LDFLAGS set on Mac/UNIX to in…

…stallation instructions [ci skip]

doc/source/installation.rst

@@ -62,7 +62,10 @@ or to upgrade without upgrading the dependencies::
 
 Installing with pip will automatically install the required
 dependencies (``numpy``, ``scipy``, and ``matplotlib``), but not the
-optional dependencies.
+optional dependencies. On a Mac/UNIX system, you can make sure to include 
+the necessary GSL environment variables by doing (see :ref:`below <gsl_cflags>`)::
+
+  export CFLAGS="$CFLAGS -I`gsl-config --prefix`/include" && export LDFLAGS="$LDFLAGS -L`gsl-config --prefix`/lib" && pip install galpy
 
 Latest version
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@@ -310,9 +313,17 @@ to add the ``'-I/usr/include'`` and ``'-L/usr/lib'`` to them.
 If you are on a Mac or UNIX system (e.g., Linux), you can find the correct ``CFLAGS`` and ``LDFLAGS``/``LD_LIBRARY_path`` entries by doing::
 
    gsl-config --cflags
-   gsl-config --libs
+   gsl-config --prefix
+
+where you should add ``/lib`` to the output of the latter. In a bash shell, you could also simply do::
+
+   export CFLAGS="$CFLAGS -I`gsl-config --prefix`/include" && export LDFLAGS="$LDFLAGS -L`gsl-config --prefix`/lib" && pip install galpy
+   
+or::
+
+   export CFLAGS="$CFLAGS -I`gsl-config --prefix`/include" && export LDFLAGS="$LDFLAGS -L`gsl-config --prefix`/lib" && python setup.py install
 
-(don't include the ``-lgsl lgslcblas`` portion of the latter output.)
+depending on whether you are installing using ``pip`` or from source.
 
 I have defined ``CFLAGS``, ``LDFLAGS``, and ``LD_LIBRARY_PATH``, but the compiler does not seem to include these and still returns with errors
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