update build docs (thanks @vinniec2)

Author: electronstudio

BUILDING.rst

@@ -4,7 +4,8 @@ Building from source
 Have Pip build from source
 --------------------------
 
-This is useful if the binaries don’t work on your system, or you want to use a newer version of Raylib.
+This is useful if the binaries don’t work on your system, or you want to use a newer version of Raylib, or compile
+Raylib with a different version of OpenGL than the default.  (e.g. on old hardware use cmake option ``-DOPENGL_VERSION=1.1``)
 
 First make sure Raylib is installed.  On Linux/Mac it must include the pkg-config files.  Best way to ensure this
 is to compile and install Raylib using CMake: https://github.com/raysan5/raylib/wiki/Working-on-GNU-Linux#build-raylib-using-cmake
@@ -13,21 +14,47 @@ Requirements for build: cmake, pkg-config.
 
 ::
 
-      cd raylib-5.0
-      mkdir build
-      cd build
-      cmake  -DCUSTOMIZE_BUILD=ON -DSUPPORT_FILEFORMAT_JPG=ON -DSUPPORT_FILEFORMAT_FLAC=ON -DWITH_PIC=ON -DCMAKE_BUILD_TYPE=Release ..
-      make
-      sudo make install
-
-
+        wget https://github.com/raysan5/raylib/archive/refs/tags/5.5.zip
+        unzip 5.5.zip
+        cd raylib-5.5
+        mkdir build
+        cd build
+        cmake  -DCUSTOMIZE_BUILD=ON -DSUPPORT_FILEFORMAT_JPG=ON -DSUPPORT_FILEFORMAT_FLAC=ON -DWITH_PIC=ON -DCMAKE_BUILD_TYPE=Release ..
+        make
+        sudo make install
+        export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
 
 Then ask Pip to build from source:
 
 ::
 
    pip3 install --no-cache-dir --no-binary raylib --upgrade --force-reinstall raylib
 
+Environment variables (new in 5.5.0.3)
+--------------------------------------
+
+If plg-config doesn't work you can manually set these environment variables:
+
+``RAYLIB_PLATFORM``: Any one of: ``Desktop``, ``SDL``, ``DRM``, ``PLATFORM_COMMA``
+
+``RAYLIB_LINK_ARGS``: Arguments to pass to the linker rather than getting them from pkg-config.
+e.g.: ``"-L/usr/local/lib -lraylib"``
+
+``RAYLIB_INCLUDE_PATH``: Directory to find raylib.h rather than getting from pkg-config.
+e.g.: ``/usr/local/include``
+
+``RAYGUI_INCLUDE_PATH``: Directory to find raygui.h
+e.g.: ``/usr/local/include``
+
+``GLFW_INCLUDE_PATH``: Directory to find glfw3.h
+e.g.: ``/usr/local/include/GLFW``
+
+``PHYSAC_INCLUDE_PATH``: Directory to find physac.h
+e.g.: ``/usr/local/include``
+
+``LIBFFI_INCLUDE_PATH``:
+e.g.: ``/usr/local/include``
+
 Or, Build from source manually
 ------------------------------
 
@@ -116,6 +143,7 @@ Build and install Raylib from the raylib-c directory.
    cd build
    cmake -DCUSTOMIZE_BUILD=ON -DSUPPORT_FILEFORMAT_JPG=ON -DSUPPORT_FILEFORMAT_FLAC=ON -DWITH_PIC=ON -DCMAKE_BUILD_TYPE=Release ..
    sudo make install
+   export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
 
 .. note:: Optional: Build the Raylib shared libs, if you plan to use
    ``raylib_dynamic`` binding.