Fix build build errors on AL2012, due to linking librt statically (#632)

**Issue:**

Build errors on AL2012, due to linking librt statically


**Diagnosis:**

We have a [tricky bit of code in setup.py](https://github.com/awslabs/aws-crt-python/blob/bc205fa35e28fab19cddd3b9cd506cdc031d98e9/setup.py#L351-L362) where we force SOME libraries to be linked statically. The location of this code is sensitive. Libraries added afterwards are NOT forced to be statically linked, and that's on purpose. That code was moved in a [recent PR](#630), and now it's forcing ALL libraries to be linked statically, even ones that are always available on the OS.

**Description of changes:**

* Move tricky bit of code back where it was
* Remove some old code that only applied to ancient versions of python
* Simplify some code that checks whether or not libcrypto is being used

Author: graebm

Diff for: setup.py

@@ -55,11 +55,7 @@ def run_cmd(args):
 
 
 def copy_tree(src, dst):
-    if sys.version_info >= (3, 8):
-        shutil.copytree(src, dst, dirs_exist_ok=True)
-    else:
-        shutil.rmtree(dst, ignore_errors=True)
-        shutil.copytree(src, dst)
+    shutil.copytree(src, dst, dirs_exist_ok=True)
 
 
 def is_macos_universal2():
@@ -144,6 +140,16 @@ def using_system_libs():
             or not os.path.exists(os.path.join(PROJECT_DIR, 'crt', 'aws-c-common', 'CMakeLists.txt')))
 
 
+def using_libcrypto():
+    """If true, libcrypto is used, even on win/mac."""
+    if sys.platform == 'darwin' or sys.platform == 'win32':
+        # use libcrypto on mac/win to support ed25519, unless its disabled via env-var
+        return not os.getenv('AWS_CRT_BUILD_DISABLE_LIBCRYPTO_USE_FOR_ED25519_EVERYWHERE') == '1'
+    else:
+        # on Unix we always use libcrypto
+        return True
+
+
 def using_system_libcrypto():
     """If true, don't build AWS-LC. Use the libcrypto that's already on the system."""
     return using_system_libs() or os.getenv('AWS_CRT_BUILD_USE_SYSTEM_LIBCRYPTO') == '1'
@@ -154,11 +160,6 @@ def forcing_static_libs():
     return os.getenv('AWS_CRT_BUILD_FORCE_STATIC_LIBS') == '1'
 
 
-def disable_libcrypto_use_for_ed25519_everywhere():
-    """If true, libcrypto is not used on mac/win to support ed25519 and apis instead return not supported."""
-    return os.getenv('AWS_CRT_BUILD_DISABLE_LIBCRYPTO_USE_FOR_ED25519_EVERYWHERE') == '1'
-
-
 class AwsLib:
     def __init__(self, name, extra_cmake_args=[], libname=None):
         self.name = name
@@ -170,7 +171,7 @@ def __init__(self, name, extra_cmake_args=[], libname=None):
 # They're built along with the extension (unless using_system_libs() is True)
 AWS_LIBS = []
 
-if not disable_libcrypto_use_for_ed25519_everywhere() or (sys.platform != 'darwin' and sys.platform != 'win32'):
+if using_libcrypto():
     # aws-lc produces libcrypto.a
     AWS_LIBS.append(AwsLib('aws-lc', libname='crypto'))
 
@@ -223,10 +224,10 @@ def _build_dependencies_impl(self, build_dir, install_path, osx_arch=None):
             f'-DCMAKE_BUILD_TYPE={build_type}',
         ])
 
-        if using_system_libcrypto():
-            cmake_args.append('-DUSE_OPENSSL=ON')
+        if using_libcrypto():
+            if using_system_libcrypto():
+                cmake_args.append('-DUSE_OPENSSL=ON')
 
-        if not disable_libcrypto_use_for_ed25519_everywhere():
             cmake_args.append('-DAWS_USE_LIBCRYPTO_TO_SUPPORT_ED25519_EVERYWHERE=ON')
 
         if sys.platform == 'darwin':
@@ -360,6 +361,19 @@ def awscrt_ext():
         extra_link_args += ['-framework', 'Security']
 
     else:  # unix
+        if forcing_static_libs():
+            # linker will prefer shared libraries over static if it can find both.
+            # force linker to choose static variant by using
+            # "-l:libaws-c-common.a" syntax instead of just "-laws-c-common".
+            #
+            # This helps AWS developers creating Lambda applications from Brazil.
+            # In Brazil, both shared and static libs are available.
+            # But Lambda requires all shared libs to be explicitly packaged up.
+            # So it's simpler to link them in statically and have less runtime dependencies.
+            #
+            # Don't apply this trick to dependencies that are always on the OS (e.g. librt)
+            libraries = [':lib{}.a'.format(x) for x in libraries]
+
         # OpenBSD doesn't have librt; functions are found in libc instead.
         if not sys.platform.startswith('openbsd'):
             libraries += ['rt']
@@ -386,20 +400,6 @@ def awscrt_ext():
         # Do this even if using system libcrypto, since it could still be a static lib.
         extra_link_args += ['-Wl,--exclude-libs,libcrypto.a']
 
-    if not disable_libcrypto_use_for_ed25519_everywhere() or (sys.platform != 'darwin' and sys.platform != 'win32'):
-        if forcing_static_libs():
-            # linker will prefer shared libraries over static if it can find both.
-            # force linker to choose static variant by using
-            # "-l:libaws-c-common.a" syntax instead of just "-laws-c-common".
-            #
-            # This helps AWS developers creating Lambda applications from Brazil.
-            # In Brazil, both shared and static libs are available.
-            # But Lambda requires all shared libs to be explicitly packaged up.
-            # So it's simpler to link them in statically and have less runtime dependencies.
-            #
-            # Don't apply this trick to dependencies that are always on the OS (e.g. librt)
-            libraries = [':lib{}.a'.format(x) for x in libraries]
-
     if sys.platform != 'win32' or distutils.ccompiler.get_default_compiler() != 'msvc':
         extra_compile_args += ['-Wno-strict-aliasing', '-std=gnu99']