link OpenMP explicitly; help CMake actually find it

recipe/bld.bat

@@ -1,4 +1,5 @@
 @echo on
+SetLocal EnableDelayedExpansion
 
 :: show CPU arch to detect slow CI agents early (rather than wait for 6h timeout)
 python -c "import numpy; numpy.show_config()"
@@ -9,6 +10,14 @@ copy %BUILD_PREFIX%\Library\bin\flang-new.exe %BUILD_PREFIX%\Library\bin\flang.e
 mkdir build
 cd build
 
+if "%USE_OPENMP%"=="1" (
+    REM https://discourse.cmake.org/t/how-to-find-openmp-with-clang-on-macos/8860
+    set "CMAKE_EXTRA=-DOpenMP_ROOT=%LIBRARY_LIB%"
+    REM not picked up by `find_package(OpenMP)` for some reason
+    set "CMAKE_EXTRA=-DOpenMP_Fortran_FLAGS=-fopenmp -DOpenMP_Fortran_LIB_NAMES=libomp"
+    set "FFLAGS=%FFLAGS% -I%LIBRARY_INC%"
+)
+
 :: millions of lines of warnings with clang-19
 set "CFLAGS=%CFLAGS% -w"
 
@@ -23,6 +32,7 @@ cmake -G "Ninja"                            ^
     -DNUM_THREADS=128                       ^
     -DBUILD_SHARED_LIBS=on                  ^
     -DUSE_OPENMP=%USE_OPENMP%               ^
+    !CMAKE_EXTRA!                           ^
     %SRC_DIR%
 if %ERRORLEVEL% neq 0 exit 1
 

recipe/meta.yaml

@@ -8,6 +8,8 @@ package:
 source:
   url: https://github.com/OpenMathLib/OpenBLAS/archive/v{{ version }}.tar.gz
   sha256: aa2d68b1564fe2b13bc292672608e9cdeeeb6dc34995512e65c3b10f4599e897
+  patches:
+    - patches/0001-explicitly-link-to-OpenMP.patch
 
 build:
   number: {{ build_number }}

recipe/patches/0001-explicitly-link-to-OpenMP.patch

@@ -0,0 +1,51 @@
+From 4cab116c4e811f977c816f3027df57d1fba3bd0c Mon Sep 17 00:00:00 2001
+From: "H. Vetinari" <h.vetinari@gmx.com>
+Date: Mon, 1 Jul 2024 09:05:00 +1100
+Subject: [PATCH] explicitly link to OpenMP
+
+bump CMake minimum to adopt CMP0074 for checking OpenMP_ROOT, see
+https://cmake.org/cmake/help/latest/policy/CMP0074.html
+---
+ CMakeLists.txt | 15 ++++++++++++++-
+ 1 file changed, 14 insertions(+), 1 deletion(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 096ca88b3..ef505f70d 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -2,7 +2,7 @@
+ ## Author: Hank Anderson <hank@statease.com>
+ ##
+
+-cmake_minimum_required(VERSION 2.8.5)
++cmake_minimum_required(VERSION 3.12)
+
+ project(OpenBLAS C ASM)
+
+@@ -100,6 +100,10 @@ endif()
+
+ message(WARNING "CMake support is experimental. It does not yet support all build options and may not produce the same Makefiles that OpenBLAS ships with.")
+
++if (USE_OPENMP)
++  find_package(OpenMP REQUIRED)
++endif ()
++
+ include("${PROJECT_SOURCE_DIR}/cmake/utils.cmake")
+ include("${PROJECT_SOURCE_DIR}/cmake/system.cmake")
+
+@@ -256,6 +260,15 @@ if (${CMAKE_SYSTEM_NAME} MATCHES "AIX|Android|Linux|FreeBSD|OpenBSD|NetBSD|Drago
+   endif()
+ endif()
+
++if (USE_OPENMP)
++  if(BUILD_STATIC_LIBS)
++    target_link_libraries(${OpenBLAS_LIBNAME}_static OpenMP::OpenMP_C)
++  endif()
++  if(BUILD_SHARED_LIBS)
++    target_link_libraries(${OpenBLAS_LIBNAME}_shared OpenMP::OpenMP_C)
++  endif()
++endif()
++
+ # Seems that this hack doesn't required since macOS 11 Big Sur
+ if (APPLE AND BUILD_SHARED_LIBS AND CMAKE_HOST_SYSTEM_VERSION VERSION_LESS 20)
+   set (CMAKE_C_USE_RESPONSE_FILE_FOR_OBJECTS 1)