ld: library not found for -lstdc++.6

You'll have this issue when targeting iOS App. It's stated in the Release note:

Deprecation Notices:

Building with libstdc++ was deprecated with Xcode 8 and is not supported in Xcode 10 when targeting iOS. C++ projects must now migrate to libc++ and are recommended to set a deployment target of iOS 7 or later. Besides changing the C++ Standard Library build setting, developers should audit hard-coded linker flags and target dependencies to remove references to libstdc++ (including -lstdc++, -lstdc++.6.0.9, libstdc++.6.0.9.tbd, and libstdc++.6.0.9.dylib). Project dependencies such as static archives that were built against libstdc++ will also need to be rebuilt against libc++. (40885260)

Source: Release Notes of XCode Beta 2

Side Note:
You need to be logged to access the page.
Link might break in next beta release (URLs change), but it's in the part Developers/Download

XCode 10 being officially released with its release note, it's still as such:

Building with libstdc++ was deprecated with Xcode 8 and is not supported in Xcode 10 when targeting iOS. C++ projects must now migrate to libc++ and are recommended to set a deployment target of macOS 10.9 or later, or iOS 7 or later. Besides changing the C++ Standard Library build setting, developers should audit hard-coded linker flags and target dependencies to remove references to libstdc++ (including -lstdc++, -lstdc++.6.0.9, libstdc++.6.0.9.tbd, and libstdc++.6.0.9.dylib). Project dependencies such as static archives that were built against libstdc++ will also need to be rebuilt against libc++. (40885260)

The quick solution is to copy all libstdc++.* file from old Xcode(9.4) to new Xcode(10.x)

For device :

cp /Applications/Xcode9.4.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/libstdc++.* /Applications/Xcode10.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/

For Simulator :

cp /Applications/Xcode9.4.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/lib/libstdc++.* /Applications/Xcode10.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/lib/

As said above lstdc++ is removed from Xcode 10. To fix this,

1. Go to Target -> BuildPhases -> Link Binary With Libraries

2. Search for lstdc++ and remove it.

3. Now you might get error in some framework which uses the above said "lstdc++" library. Now either you have to update those framework or remove it so that Xcode can build it successfully.