Implemented get and set environment variables for posix.

src/main/cpp/posix.cpp

@@ -168,6 +168,34 @@ Java_net_rubygrapefruit_platform_internal_jni_PosixProcessFunctions_setWorkingDi
     free(path);
 }
 
+JNIEXPORT jstring JNICALL
+Java_net_rubygrapefruit_platform_internal_jni_PosixProcessFunctions_getEnvironmentVariable(JNIEnv *env, jclass target, jstring var, jobject result) {
+    char* varStr = java_to_char(env, var, result);
+    char* valueStr = getenv(varStr);
+    free(varStr);
+    if (valueStr == NULL) {
+        return NULL;
+    }
+    return char_to_java(env, valueStr, result);
+}
+
+JNIEXPORT void JNICALL
+Java_net_rubygrapefruit_platform_internal_jni_PosixProcessFunctions_setEnvironmentVariable(JNIEnv *env, jclass target, jstring var, jstring value, jobject result) {
+    char* varStr = java_to_char(env, var, result);
+    if (value == NULL) {
+        if (setenv(varStr, "", 1) != 0) {
+            mark_failed_with_errno(env, "could not putenv()", result);
+        }
+    } else {
+        char* valueStr = java_to_char(env, value, result);
+        if (setenv(varStr, valueStr, 1) != 0) {
+            mark_failed_with_errno(env, "could not putenv()", result);
+        }
+        free(valueStr);
+    }
+    free(varStr);
+}
+
 /*
  * Terminal functions
  */