Add AWS-LC src & ref flags to Gradle build (#390)

build.gradle

@@ -20,6 +20,12 @@ if (ext.isFips) {
 } else {
     ext.awsLcGitVersionId = 'v1.30.1'
 }
+
+// Check for user inputted git version ID.
+if (System.properties["AWSLC_GITVERSION"]) {
+    ext.awsLcGitVersionId = System.properties["AWSLC_GITVERSION"]
+}
+
 ext.isLegacyBuild = Boolean.getBoolean('LEGACY_BUILD')
 
 ext.lcovIgnore = System.properties['LCOV_IGNORE']
@@ -90,6 +96,11 @@ def getClangFormatVersion() {
 
 def awslcSrcPath = "${projectDir}/aws-lc/"
 
+// Check for user inputted AWS-LC source directory. 
+if (System.properties["AWSLC_SRC_DIR"]) {
+    awslcSrcPath = System.properties["AWSLC_SRC_DIR"]
+}
+
 // Execute cmake3 command to see if it exists. Mainly to support AL2.
 def detect_cmake3 = {
     def exec_cmake3 = exec {
@@ -174,7 +185,11 @@ task buildAwsLc {
 
     doFirst {
         if (file(awslcSrcPath).list().size() == 0) {
-            throw new GradleException("aws-lc dir empty! run 'git submodule update --init --recursive' to populate.")
+            if (System.properties["AWSLC_SRC_DIR"]) {
+                throw new GradleException("aws-lc dir empty! specify another directory or populate with aws-lc source files.")
+            } else {
+                throw new GradleException("aws-lc dir empty! run 'git submodule update --init --recursive' to populate.")
+            }
         }
 
         if (!isLegacyBuild) {