configure: Automatically add -isysroot for darwin if --sysroot is specified

Check the existing flags in the cc/cflags/cppflags/ldflags for
occurrances of -isysroot; if none is found but --sysroot was specified,
set -isysroot to the same value as --sysroot.

This simplifies configuring cross-builds for iOS, if the global
environment variable SDKROOT isn't set.

Signed-off-by: Martin Storsjö <martin@martin.st>