openblas: add patches to build with clang (#39138)

var/spack/repos/builtin/packages/openblas/openblas_append_lflags.patch

@@ -0,0 +1,26 @@
+diff --git a/ctest/Makefile b/ctest/Makefile
+index 0fb2450d28..9e85d23b96 100644
+--- a/ctest/Makefile
++++ b/ctest/Makefile
+@@ -208,7 +208,7 @@ FLDFLAGS = $(FFLAGS:-fPIC=) $(LDFLAGS)
+ ifeq ($(USE_OPENMP), 1)
+ ifeq ($(F_COMPILER), GFORTRAN)
+ ifeq ($(C_COMPILER), CLANG)
+-CEXTRALIB = -lomp
++CEXTRALIB += -lomp
+ endif
+ endif
+ ifeq ($(F_COMPILER), NAG)
+diff --git a/test/Makefile b/test/Makefile
+index 923f1537c2..46a7b11583 100644
+--- a/test/Makefile
++++ b/test/Makefile
+@@ -265,7 +265,7 @@ FLDFLAGS = $(FFLAGS:-fPIC=) $(LDFLAGS)
+ ifeq ($(USE_OPENMP), 1)
+ ifeq ($(F_COMPILER), GFORTRAN)
+ ifeq ($(C_COMPILER), CLANG)
+-CEXTRALIB = -lomp
++CEXTRALIB += -lomp
+ endif
+ endif
+ ifeq ($(F_COMPILER), NAG)

var/spack/repos/builtin/packages/openblas/openblas_libomp_fork.patch

@@ -0,0 +1,13 @@
+diff --git a/utest/Makefile b/utest/Makefile
+index f99035440c..eab9046574 100644
+--- a/utest/Makefile
++++ b/utest/Makefile
+@@ -29,8 +29,11 @@ ifeq ($(OSNAME), $(filter $(OSNAME),Linux CYGWIN_NT))
+ ifneq ($(USE_OPENMP), 1)
+ OBJS += test_fork.o
+ endif
++# LLVM's libomp throws an assertion when running the post fork test
++ifneq ($(C_COMPILER), CLANG)
+ OBJS += test_post_fork.o
+ endif
++endif

var/spack/repos/builtin/packages/openblas/package.py

@@ -195,6 +195,17 @@ class Openblas(CMakePackage, MakefilePackage):
         when="@0.3.21 %gcc@:9",
     )
 
+    # Some installations of clang and libomp have non-standard locations for
+    # libomp. OpenBLAS adds the correct linker flags but overwrites the
+    # variables in a couple places, causing link-time failures.
+    patch("openblas_append_lflags.patch", when="@:0.3.23 threads=openmp")
+
+    # Some builds of libomp on certain systems cause test failures related to
+    # forking, so disable the specific test that's failing. This is currently
+    # an open issue upstream:
+    # https://github.com/llvm/llvm-project/issues/63908
+    patch("openblas_libomp_fork.patch", when="%clang@15:")
+
     # Fix build on A64FX for OpenBLAS v0.3.24
     patch(
         "https://github.com/OpenMathLib/OpenBLAS/commit/90231bfc4e4afc51f67c248328fbef0cecdbd2c2.patch?full_index=1",