support in source builds of aws-c-* libs. (#566)

* support in source builds of aws-c-* libs.

CMakeLists.txt

@@ -219,6 +219,7 @@ list(APPEND EXPORT_MODULES
     "cmake/AwsLibFuzzer.cmake"
     "cmake/AwsSanitizers.cmake"
     "cmake/AwsSIMD.cmake"
+    "cmake/AwsFindPackage.cmake"
     )
 
 install(FILES ${EXPORT_MODULES}

cmake/AwsFindPackage.cmake

@@ -0,0 +1,32 @@
+# Copyright 2010-2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License").
+# You may not use this file except in compliance with the License.
+# A copy of the License is located at
+#
+#  http://aws.amazon.com/apache2.0
+#
+# or in the "license" file accompanying this file. This file is distributed
+# on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+# express or implied. See the License for the specific language governing
+# permissions and limitations under the License.
+
+option(IN_SOURCE_BUILD "If the CRT libs are being built from your source tree (add_subdirectory), set this to ON" OFF)
+
+# This function handles dependency list building based on if traditional CMAKE modules via. find_package should be
+# used, vs if this is an in source build via. something like git submodules and add_subdirectory.
+# This is largely because CMake was not well planned out, and as a result, in-source and modules don't play well
+# together. Only use this on CRT libraries (including S2N), libcrypto will stay as an assumed external dependency.
+#
+# package_name: is the name of the package to find
+# DEP_CRT_LIBS: output variable will be appended after each call to this function. You don't have to use it,
+#    but it can be passed directly target_link_libraries and it will be the properly qualified library
+#    name and namespace based on configuration.
+function(aws_use_package package_name)
+    if (IN_SOURCE_BUILD)
+        set(DEP_AWS_LIBS ${DEP_AWS_LIBS} ${package_name} PARENT_SCOPE)
+    else()
+        find_package(${package_name} REQUIRED)
+        set(DEP_AWS_LIBS ${DEP_AWS_LIBS} AWS::${package_name} PARENT_SCOPE)
+    endif()
+endfunction()