Fix build of llvmjit_types.bc with meson

When building on mac with both NLS and LLVM enabled using meson:

  meson setup --prefix=$HOME/.local/ -Dbuildtype=debug -Dllvm=enabled -Dicu=enabled -Dnls=enabled -Dextra_lib_dirs=/opt/homebrew/lib -Dextra_include_dirs=/opt/homebrew/include ../

Compilation fails with the following error:

clang -c -o src/backend/jit/llvm/llvmjit_types.bc ../src/backend/jit/llvm/llvmjit_types.c -flto=thin -emit-llvm -MD -MQ src/backend/jit/llvm/llvmjit_types.bc -MF src/backend/jit/llvm/llvmjit_types.c.bc.d -O2 -Wno-ignored-attributes -Wno-empty-body -fno-strict-aliasing -fwrapv -I/opt/homebrew/opt/openssl@3/include -I./src/include -I./src/backend/utils/misc -I../src/include
In file included from ../src/backend/jit/llvm/llvmjit_types.c:27:
In file included from ../src/include/postgres.h:48:
../src/include/c.h:82:10: fatal error: 'libintl.h' file not found
   82 | #include <libintl.h>
      |          ^~~~~~~~~~~

The failure happens in the custom_target llvmjit_types.bc. The missing libintl.h header is available in /opt/homebrew/include/libintl.h which is provided as an extra_include_dirs. However, it won't be used by the custom_target which hardcodes the include path.

This patch modifies llvmjit_types.bc's include paths to use postgres_inc_d which includes paths provided in extra_include_dirs. 

The code relies on meson's '/' behaviour[1] which drops all segments before an absolute path. With /opt/homebrew/include as an extra include dir, 'meson.project_source_root() / inc_d' will yield /opt/homebrew/include.

I've also removed the include of src/backend/utils/misc which doesn't seem necessary? The only header there is guc_internal.h which is only used by files in the same directory.

BUILD_ROOT and SOURCE_ROOT were also replaced by meson.project_build_root and meson.project_source_root. BUILD_ROOT and SOURCE_ROOT provide the source root of the main project if called from a subproject, preventing the use of postgres as a subproject.

If you test with LLVM21, you will need to apply the patch provided in [2] as compilation will fail without this patch.

[1]: https://mesonbuild.com/Syntax.html#string-path-building

[2]: https://www.postgresql.org/message-id/flat/44718204-61a5-91b1-5574-154ff7a019f6%40applied-asynchrony.com#9a64b5e537204aa2460c4b0d008048b9