Replace CMAKE_(SOURCE|BINARY)_DIR with CMAKE_CURRENT_(SOURCE|BINARY)_DIR

It is possible to add a subproject using add_subdirectory command of
CMake. In that particular case, CMAKE_(SOURCE|BINARY)_DIR points to the
top level project's directories while CMAKE_CURRENT_(SOURCE|BINARY)_DIR
points to Aeron's directories. Stand alone builds or adding Aeron as a
subproject using ExternalProject_Add does not get affected.

CMakeLists.txt

@@ -19,7 +19,7 @@ cmake_policy(VERSION 3.0.2)
 file(STRINGS version.txt VERSION_TXT LIMIT_COUNT 1 REGEX "^[0-9]+(\\.[0-9]+)+")
 string(REGEX REPLACE "^([0-9]+(\\.[0-9]+)+).*$" "\\1" VERSION_FROM_FILE "${VERSION_TXT}")
 
-if (${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
+if (${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
     message(FATAL_ERROR "In-source builds not allowed. Please make a new directory (called a build directory)"
         "and run CMake from there. You may need to remove CMakeCache.txt.")
 endif()
@@ -185,13 +185,13 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/binaries")
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/lib")
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/lib")
 
-set(AERON_SAMPLES_PATH "${CMAKE_SOURCE_DIR}/aeron-samples/src/main/cpp")
+set(AERON_SAMPLES_PATH "${CMAKE_CURRENT_SOURCE_DIR}/aeron-samples/src/main/cpp")
 
-set(AERON_CLIENT_SOURCE_PATH "${CMAKE_SOURCE_DIR}/aeron-client/src/main/cpp")
-set(AERON_CLIENT_TEST_PATH "${CMAKE_SOURCE_DIR}/aeron-client/src/test/cpp")
+set(AERON_CLIENT_SOURCE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/aeron-client/src/main/cpp")
+set(AERON_CLIENT_TEST_PATH "${CMAKE_CURRENT_SOURCE_DIR}/aeron-client/src/test/cpp")
 
-set(AERON_DRIVER_SOURCE_PATH "${CMAKE_SOURCE_DIR}/aeron-driver/src/main/c")
-set(AERON_DRIVER_TEST_PATH "${CMAKE_SOURCE_DIR}/aeron-driver/src/test/c")
+set(AERON_DRIVER_SOURCE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/aeron-driver/src/main/c")
+set(AERON_DRIVER_TEST_PATH "${CMAKE_CURRENT_SOURCE_DIR}/aeron-driver/src/test/c")
 
 # gmock - includes gtest
 include_directories(${GMOCK_SOURCE_DIR}/googletest/include)