fix(cmake): make dash_static find libedit headers on Linux

[lcolok]

Summary

On Linux, a clean cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build against master (72bc8c2, v2.1.0) fails to compile dash_static:

/home/logic/boxsh/third_party/dash-0.5.12/src/myhistedit.h:34:10:
fatal error: histedit.h: No such file or directory
   34 | #include <histedit.h>
      |          ^~~~~~~~~~~~
compilation terminated.

Root cause

CMakeLists.txt wires dash_static's include dirs at line 73:

target_include_directories(dash_static
    PUBLIC
        ${DASH_SRC_DIR}
        ${DASH_ROOT}
)

But LIBEDIT_INSTALL_DIR (where libedit's histedit.h lands after libedit_vendored builds) is only defined at line 116, so dash_static has no visibility into libedit's headers. On top of that, dash_static has no explicit dependency on libedit_vendored, so CMake is free to start compiling dash sources before libedit's headers are installed.

Linux reproduces this because the dash build unconditionally requires histedit.h. macOS happens to resolve the header through another search path, which is why the regression was not caught there.

Fix

Three small, additive changes to CMakeLists.txt:

1. Move set(LIBEDIT_INSTALL_DIR ${CMAKE_BINARY_DIR}/libedit-prefix) before add_library(dash_static), so the variable is defined before it is used.
2. Add ${LIBEDIT_INSTALL_DIR}/include to target_include_directories(dash_static) so #include <histedit.h> resolves.
3. Add add_dependencies(dash_static libedit_vendored) so the external libedit build (which installs histedit.h into that include dir) completes before dash_static compilation starts.

Diff is 4 lines, no logic changed elsewhere.

Verification

Tested on Ubuntu 24.04 LTS (x86_64):

With the patch — clean Release build succeeds:

$ rm -rf build && cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build -j8
...
[100%] Linking CXX executable boxsh
[100%] Built target boxsh
-rwxrwxr-x 1 logic logic 1.6M build/boxsh

Basic smoke tests all pass:

Check	Result
boxsh --version	boxsh version 2.1.0
boxsh -c 'echo hi'	hi
boxsh --sandbox -c 'cat /proc/1/comm'	boxsh (PID namespace active)
boxsh --sandbox --new-net-ns -c 'ping -c1 -W1 8.8.8.8'	Network is unreachable
--bind cow:SRC:DST writes isolated from SRC	confirmed

Reverse verification — stashing the patch reliably reproduces the failure:

$ git stash
$ rm -rf build && cmake -B build -DCMAKE_BUILD_TYPE=Release
$ cmake --build build -j8 2>&1 | grep -E 'error|histedit'
/home/logic/boxsh/third_party/dash-0.5.12/src/myhistedit.h:34:10:
fatal error: histedit.h: No such file or directory

Risk

Low. The change only reorders a CMake variable definition and adds one include path plus one add_dependencies edge for a target that already depends on libedit at link time via boxsh. No runtime behavior changes.