Add -ObjC to static podspec; update custom CocoaPods steps

[friedbunny]

Added the -ObjC flag to the static podspec, per @frederoni in #6213 (comment). This is a mixed bag (because CocoaPods is CocoaPods) and still requires the user to add an extra build flag:

[!] The `firststepsstatic [Release]` target overrides the `OTHER_LDFLAGS` build setting defined in `Pods/Target Support Files/Pods-firststepsstatic/Pods-firststepsstatic.release.xcconfig'. This can lead to problems with the CocoaPods installation
    - Use the `$(inherited)` flag, or
    - Remove the build settings from the target.

Also rewrote the custom build CocoaPods instructions to reflect the changes in #5653.

Fixes #6195, refs #6213.

/cc @1ec5 @frederoni

[mention-bot]

@friedbunny, thanks for your PR! By analyzing this pull request, we identified @1ec5, @tmcw and @boundsj to be potential reviewers.