docs: add a note about compiling the example (#2737)

* docs: mention PYTHONPATH in installing.rst

When pybind11 is included as a submodule, the user needs to update their
Python module search path.  Otherwise, the first c++ compilation command
in docs/basics.rst will fail.

* docs: add a note about compiling the example

This note shows how to modify the compilation command for the example
when the pybind11 source has been included as a Git submodule.

* docs: add a note about compiling the example

Added an internal link to the docs

* docs: updated a note about compiling the example

Also updated the command substitution syntax for consistency

docs/basics.rst

@@ -136,7 +136,14 @@ On Linux, the above example can be compiled using the following command:
 
 .. code-block:: bash
 
-    $ c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension-suffix`
+    $ c++ -O3 -Wall -shared -std=c++11 -fPIC $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
+
+.. note::
+
+    If you used :ref:`include_as_a_submodule` to get the pybind11 source, then
+    use ``$(python3-config --includes) -Iextern/pybind11/include`` instead of
+    ``$(python3 -m pybind11 --includes)`` in the above compilation, as
+    explained in :ref:`building_manually`.
 
 For more details on the required compiler flags on Linux and macOS, see
 :ref:`building_manually`. For complete cross-platform compilation instructions,

docs/compiling.rst

@@ -565,7 +565,7 @@ On Linux, you can compile an example such as the one given in
 
 .. code-block:: bash
 
-    $ c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension-suffix`
+    $ c++ -O3 -Wall -shared -std=c++11 -fPIC $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
 
 The flags given here assume that you're using Python 3. For Python 2, just
 change the executable appropriately (to ``python`` or ``python2``).
@@ -577,7 +577,7 @@ using ``pip`` or ``conda``. If it hasn't, you can also manually specify
 ``python3-config --includes``.
 
 Note that Python 2.7 modules don't use a special suffix, so you should simply
-use ``example.so`` instead of ``example`python3-config --extension-suffix```.
+use ``example.so`` instead of ``example$(python3-config --extension-suffix)``.
 Besides, the ``--extension-suffix`` option may or may not be available, depending
 on the distribution; in the latter case, the module extension can be manually
 set to ``.so``.
@@ -588,7 +588,7 @@ building the module:
 
 .. code-block:: bash
 
-    $ c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension-suffix`
+    $ c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
 
 In general, it is advisable to include several additional build parameters
 that can considerably reduce the size of the created binary. Refer to section

docs/installing.rst

@@ -8,6 +8,8 @@ There are several ways to get the pybind11 source, which lives at
 developers recommend one of the first three ways listed here, submodule, PyPI,
 or conda-forge, for obtaining pybind11.
 
+.. _include_as_a_submodule:
+
 Include as a submodule
 ======================