dep: zmqpp: don't specify namespace while inside same namespace
dep: zmqpp: don't define htonll and ntohll if already #defined

is the case with osx: defined in /usr/include/sys/_endian.h
cmake: platform: also detect clang by C_COMPILER_ID: C_COMPILER may vary
dep: zmqpp: add diff for htonll/ntohll workaround
Core/Entities: Removed Object::GetGUIDLow()
Merge pull request #13431 from bloerwald/fix_zmqpp_htonll_and_ntohll_on_osx

dep: zmqpp: fix: don't define htonll and ntohll if already #defined
Merge pull request #13432 from bloerwald/fix_compiler_detection_on_unix

cmake: platform: also detect clang by C_COMPILER_ID: C_COMPILER may vary
Core/Opcodes: Update some opcode ids to 6.0.2 from WPP
Scripts/Ulduar: Exclude Ignis' main target from the Slag Pot target selector

Closes #10516

(cherry picked from commit f4c035554fdc25d286ddc37c24e3ab43c71f1dae)
Scripts/ScarletMonastery: Summon the Headless Horseman once the Call the Headless Horseman is quest complete

By @cr4st
Closes #13273

(cherry picked from commit 22f218540bedbe43fd5fefb049561dbeccddc6c5)
DB/Achievement: Fix map conditions for achievement "He's not getting any older"

25m used to be awarded in 10m vice-versa

(cherry picked from commit 805be8ebba07c3280089a128f1764d49f6f84ab6)
DB/World: Cherry-picks

- ee623a4006009e729ed4cfdd36aa9bb1525d72a9 & 3965ee33efd8751d64da6352227650fb4181fe31
- 8274caeef052ffaf21c9109de13b8c2a5355d9e4
- 805be8ebba07c3280089a128f1764d49f6f84ab6
- d680e82c38068d9f69393f9d32b3f10fc9729460
DB/World: Remove duplicated file added in 24bd4592fce97
Fixed gcc build
Fixed gcc build2
Fixed gcc linking
Core/Entities: First step to 128 bit guids

* Database fields storing full guid have been converted to BINARY(16)
Core/Misc: Optimized ByteBuffer::ReadString
Core/Entities: Second part of required database changes for migrating guids to 128 bit - all fields storing lowguid must be extended to uint64
Core/Entities: Added type definition to use for a low guids