Fix build failure with GCC 11:

------
[...]
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:60:51: error: ‘this’ pointer is null [-Werror=nonnull]
   60 |             os << "width=" << packedImg->getWidth() << ", ";
      |                                                   ^
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:274:10: note: in a call to non-static member function ‘long int OpenColorIO::v1::PackedImageDesc::getWidth() const’
  274 |     long PackedImageDesc::getWidth() const
      |          ^~~~~~~~~~~~~~~
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:61:53: error: ‘this’ pointer is null [-Werror=nonnull]
   61 |             os << "height=" << packedImg->getHeight() << ", ";
      |                                                     ^
/tmp/guix-build-opencolorio-1.1.1.drv-0/source/src/core/ImageDesc.cpp:279:10: note: in a call to non-static member function ‘long int OpenColorIO::v1::PackedImageDesc::getHeight() const’
  279 |     long PackedImageDesc::getHeight() const
      |          ^~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
[...]
------

Patch copied from Gentoo:

https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=7e726d850502018b6760da78dbd4a419603016b8

diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 1eb691b6..cff9bd83 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -23,8 +23,6 @@ if(WIN32)
     if("${CMAKE_BUILD_TYPE}" STREQUAL "Release")
         set(EXTERNAL_COMPILE_FLAGS "${EXTERNAL_COMPILE_FLAGS} /WX")
     endif()
-else()
-    set(EXTERNAL_COMPILE_FLAGS "${EXTERNAL_COMPILE_FLAGS} -Werror")
 endif()
 
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<tr><td><span title='2021-11-16 14:34:28 +0100'>2021-11-16</span></td><td><a href='/guix/commit/nix/libstore/local-store.hh?id=472a0e82a52a3d5d841e1dfad6b13e26082a5750'>daemon: Do not deduplicate files smaller than 8 KiB.</a><span class='msg-avail'>...<span class='msg-tooltip'>Files smaller than 8 KiB typically represent ~70% of the entries in
/gnu/store/.links but only contribute to ~4% of the space savings
afforded by deduplication.

Not considering these files for deduplication speeds up file insertion
in the store and, more importantly, leaves 'removeUnusedLinks' with
fewer entries to traverse, thereby speeding it up proportionally.

Partly fixes &lt;https://issues.guix.gnu.org/24937&gt;.

* config-daemon.ac: Remove symlink hard link check and CAN_LINK_SYMLINK
definition.
* guix/store/deduplication.scm (%deduplication-minimum-size): New
variable.
(deduplicate)[loop]: Do not recurse when FILE's size is below
%DEDUPLICATION-MINIMUM-SIZE.
(dump-port): New procedure.
(dump-file/deduplicate)[hash]: Turn into...
[dump-and-compute-hash]: ... this thunk.
Call 'deduplicate' only when SIZE is greater than
%DEDUPLICATION-MINIMUM-SIZE; otherwise call 'dump-port'.
* nix/libstore/gc.cc (LocalStore::removeUnusedLinks): Drop files where
st.st_size &lt; deduplicationMinSize.
* nix/libstore/local-store.hh (deduplicationMinSize): New declaration.
* nix/libstore/optimise-store.cc (deduplicationMinSize): New variable.
(LocalStore::optimisePath_): Return when PATH is a symlink or smaller
than 'deduplicationMinSize'.
* tests/derivations.scm ("identical files are deduplicated"): Produce
files bigger than %DEDUPLICATION-MINIMUM-SIZE.
* tests/nar.scm ("restore-file-set with directories (signed, valid)"):
Likewise.
* tests/store-deduplication.scm ("deduplicate, below %deduplication-minimum-size"):
New test.
("deduplicate", "deduplicate, ENOSPC"): Produce files bigger than
%DEDUPLICATION-MINIMUM-SIZE.
* tests/store.scm ("substitute, deduplication"): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>