STDEXEC_MAIN_PROJECT always OFF — install targets never run, breaking Conan packaging

[solarispika]

Description

Commit c9a1dc5 introduced STDEXEC_MAIN_PROJECT and STDEXEC_INSTALL to make install targets opt-out when stdexec is used as a subproject. The intent is correct, but the top-level project detection has a bug that causes STDEXEC_INSTALL to always be OFF, so cmake --install copies nothing.

Root Cause

The detection logic in CMakeLists.txt:

if (NOT DEFINED STDEXEC_MAIN_PROJECT)
  set(STDEXEC_MAIN_PROJECT OFF)
  if (NOT DEFINED PROJECT_NAME)
    set(STDEXEC_MAIN_PROJECT ON)
  endif ()
endif ()

This check runs after project(STDEXEC ...) has already been called. At that point PROJECT_NAME is always defined as "STDEXEC", so the NOT DEFINED PROJECT_NAME branch is never taken and STDEXEC_MAIN_PROJECT is always OFF.

As a result STDEXEC_INSTALL defaults to OFF and all install() rules are skipped — even when stdexec is the top-level project.

Impact

Conan's package() step calls cmake --install, which now installs nothing. Headers are not copied into the package, making the Conan package broken.

Suggested Fix

Use the standard CMake idiom to detect top-level project instead:

if (NOT DEFINED STDEXEC_MAIN_PROJECT)
  if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
    set(STDEXEC_MAIN_PROJECT ON)
  else()
    set(STDEXEC_MAIN_PROJECT OFF)
  endif()
endif()

Or equivalently, move the PROJECT_NAME check to before the project() call.

cc @mathisloge — could you take a look? You may have a better sense of the intended behavior here.