[recipe-stl] Add android's STL lib support to Recipe

To allow us to refactor some common operations that we use in our recipes that depends on android's STL library.

Note: This commit will allow us to begin the migration to `c++_shared`. This is a must when we move to android's NDK r19+, because as for android NDK >= 18 is the only one supported STL library.

Author: opacam

pythonforandroid/recipe.py

@@ -125,6 +125,46 @@ class Recipe(with_metaclass(RecipeMeta)):
               path: `'.', None or ''`
     """
 
+    need_stl_shared = False
+    '''Some libraries or python packages may need to be linked with android's
+    stl. We can automatically do this for any recipe if we set this property to
+    `True`'''
+
+    stl_lib_name = 'c++_shared'
+    '''
+    The default STL shared lib to use: `c++_shared`.
+
+    .. note:: Android NDK version > 17 only supports 'c++_shared', because
+        starting from NDK r18 the `gnustl_shared` lib has been deprecated.
+    '''
+
+    stl_lib_source = '{ctx.ndk_dir}/sources/cxx-stl/llvm-libc++'
+    '''
+    The source directory of the selected stl lib, defined in property
+    `stl_lib_name`
+    '''
+
+    @property
+    def stl_include_dir(self):
+        return join(self.stl_lib_source.format(ctx=self.ctx), 'include')
+
+    def get_stl_lib_dir(self, arch):
+        return join(
+            self.stl_lib_source.format(ctx=self.ctx), 'libs', arch.arch
+        )
+
+    def get_stl_library(self, arch):
+        return join(
+            self.get_stl_lib_dir(arch),
+            'lib{name}.so'.format(name=self.stl_lib_name),
+        )
+
+    def install_stl_lib(self, arch):
+        if not self.ctx.has_lib(
+            arch.arch, 'lib{name}.so'.format(name=self.stl_lib_name)
+        ):
+            self.install_libs(arch, self.get_stl_library(arch))
+
     @property
     def version(self):
         key = 'VERSION_' + self.name
@@ -454,7 +494,22 @@ def get_recipe_env(self, arch=None, with_flags_in_cc=True):
         """
         if arch is None:
             arch = self.filtered_archs[0]
-        return arch.get_env(with_flags_in_cc=with_flags_in_cc)
+        env = arch.get_env(with_flags_in_cc=with_flags_in_cc)
+
+        if self.need_stl_shared:
+            env['CPPFLAGS'] = env.get('CPPFLAGS', '')
+            env['CPPFLAGS'] += ' -I{}'.format(self.stl_include_dir)
+
+            env['CXXFLAGS'] = env['CFLAGS'] + ' -frtti -fexceptions'
+
+            if with_flags_in_cc:
+                env['CXX'] += ' -frtti -fexceptions'
+
+            env['LDFLAGS'] += ' -L{}'.format(self.get_stl_lib_dir(arch))
+            env['LIBS'] = env.get('LIBS', '') + " -l{}".format(
+                self.stl_lib_name
+            )
+        return env
 
     def prebuild_arch(self, arch):
         '''Run any pre-build tasks for the Recipe. By default, this checks if
@@ -538,6 +593,9 @@ def postbuild_arch(self, arch):
         if hasattr(self, postbuild):
             getattr(self, postbuild)()
 
+        if self.need_stl_shared:
+            self.install_stl_lib(arch)
+
     def prepare_build_dir(self, arch):
         '''Copies the recipe data into a build dir for the given arch. By
         default, this unpacks a downloaded recipe. You should override