abseil: disable CMAKE_INCLUDE_CURRENT_DIR for the in-tree build

[jcelerier]

Problem

Building a consumer that enables CMAKE_INCLUDE_CURRENT_DIR (e.g. ossia score, which sets it globally) fails when compiling the in-tree Abseil with:

.../include/c++/14/ctime:60:11: error: no member named 'clock_t' in the global namespace; did you mean '__clock_t'?
   60 |   using ::clock_t;

Root cause

CMAKE_INCLUDE_CURRENT_DIR ON makes CMake add every target's own source/binary dir as a plain -I. For Abseil's absl/time target this injects:

-I.../abseil-cpp/absl/time

That directory contains a header literally named time.h (absl::Time). Being a plain -I, it sits ahead of /usr/include, so when a system header does #include <time.h> (e.g. pthread.h), the compiler resolves it to Abseil's absl/time/time.h instead of the C <time.h>. The C global-namespace symbols (::clock_t, ::clock, …) are then never declared, and <ctime>'s using ::clock_t; fails.

Fix

Set CMAKE_INCLUDE_CURRENT_DIR OFF inside the existing block() scope in cmake/deps/abseil.cmake, so it applies only to the Abseil subtree. Abseil includes its own headers as absl/... (via the -isystem root) and does not rely on this setting, so the change is safe and scoped.

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