ci: fix Ubuntu 24.04 — remove libgomp1-dev (non-existent), make OpenMP optional

libgomp1-dev does not exist as a separate package on Ubuntu 24.04;
OpenMP runtime (libgomp1) and headers (omp.h) are bundled with GCC.

- ci.yml: remove libgomp1 + libgomp1-dev from apt-get install
- CMakeLists.txt: find_package(OpenMP) without REQUIRED; conditional
  target_link_libraries so build succeeds even without OpenMP
- einsum.h already guards #include <omp.h> and #pragma omp behind #ifdef _OPENMP

Author: peng.li24

.github/workflows/ci.yml

@@ -46,7 +46,9 @@ jobs:
       - name: Install system dependencies
         run: |
           sudo apt-get update -qq
-          sudo apt-get install -y libeigen3-dev libgomp1 libgomp1-dev
+          sudo apt-get install -y libeigen3-dev
+          # libgomp1 (OpenMP runtime) and omp.h (OpenMP headers) are bundled
+          # with GCC on Ubuntu 24.04 — no separate libgomp1-dev package exists
 
       - name: Verify AVX-512 support
         run: |

tests/CMakeLists.txt

@@ -39,8 +39,8 @@ endif()
 # Eigen3 (optional — used for norm/std helpers in module.cpp)
 find_package(Eigen3 CONFIG QUIET)
 
-# OpenMP
-find_package(OpenMP REQUIRED)
+# OpenMP (optional — einsum uses #pragma omp only when _OPENMP is defined)
+find_package(OpenMP)
 
 # ---- Python extension module -------------------------------------------------
 pybind11_add_module(numpycpp MODULE module.cpp)
@@ -68,7 +68,10 @@ target_compile_options(numpycpp PRIVATE
     -fno-builtin-cbrt  -fno-builtin-expm1 -fno-builtin-log1p
 )
 
-target_link_libraries(numpycpp PRIVATE OpenMP::OpenMP_CXX dl)
+if(OpenMP_CXX_FOUND)
+    target_link_libraries(numpycpp PRIVATE OpenMP::OpenMP_CXX)
+endif()
+target_link_libraries(numpycpp PRIVATE dl)
 
 # Place .so next to the test scripts for easy import
 set_target_properties(numpycpp PROPERTIES