Travis CI: build, don't download, external GLFW

CI is failing because we still test with GLFW 3.2, while
the internal GLFW is 3.3. To avoid such breakages in future,
build and install our own GLFW instead of relying on an external
Debian package.

.travis.yml

@@ -93,9 +93,16 @@ before_install:
 
         export RAYLIB_PACKAGE_SUFFIX="-Linux-$ARCH";
         if [ "$INSTALL_GLFW" == "YES" ]; then
-          wget 'http://ftp.de.debian.org/debian/pool/main/g/glfw3/libglfw3_3.2.1-1_amd64.deb';
-          wget 'http://ftp.de.debian.org/debian/pool/main/g/glfw3/libglfw3-dev_3.2.1-1_amd64.deb';
-          sudo dpkg -i libglfw3_3.2.1-1_amd64.deb libglfw3-dev_3.2.1-1_amd64.deb;
+          pushd src/external/glfw;
+          cmake . -DGLFW_BUILD_DOCS=OFF
+                  -DGLFW_BUILD_TESTS=OFF
+                  -DGLFW_BUILD_EXAMPLES=OFF
+                  -DGLFW_INSTALL=ON
+                  -DBUILD_SHARED_LIBS=ON
+                  -DGLFW_USE_WAYLAND=$WAYLAND;
+          make;
+          sudo make install;
+          popd;
         fi;
       fi;
     fi