LLVMFlang: auto-determine new or old flang runtime library

Author: scivision

README.md

@@ -129,15 +129,16 @@ The "error" examples show this.
 
 ### ISO_Fortran_binding.h
 
-On 2019-01-12 [a GCC commit](https://github.com/gcc-mirror/gcc/commit/bbf18dc5d248a79a20ebf4b3a751669cd75485fd)
+On 2019-01-12
+[a GCC commit](https://github.com/gcc-mirror/gcc/commit/bbf18dc5d248a79a20ebf4b3a751669cd75485fd)
 from Paul Thomas brought ISO_Fortran_binding.h to
 [GCC 9](https://gcc.gnu.org/gcc-9/changes.html).
 
 Compilers having "ISO_Fortran_binding.h" include:
 
 * GCC ≥ 9
 * Intel oneAPI
-* Clang / Flang LLVM ≥ 19
+* Clang / Flang LLVM ≥ 19 (note for macos: not AppleClang yet, need to use Homebrew Clang + Flang)
 * IBM OpenXL
 * Cray Fortran
 

cmake/compilers.cmake

@@ -70,7 +70,16 @@ if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
 elseif(CMAKE_C_COMPILER_ID STREQUAL "NVHPC")
   set(CMAKE_REQUIRED_LIBRARIES nvf)
 elseif(CMAKE_Fortran_COMPILER_ID STREQUAL "LLVMFlang")
-  set(CMAKE_REQUIRED_LIBRARIES FortranRuntime FortranDecimal)
+  find_library(flang_rt
+  NAMES flang_rt.runtime FortranRuntime
+  NAMES_PER_DIR
+  )
+  # newer Flang versions have flang_rt.runtime, older Flang have FortranRuntime
+  if(flang_rt)
+    set(CMAKE_REQUIRED_LIBRARIES ${flang_rt})
+  else()
+    message(WARNING "Couldn't find Flang runtime library; some checks may fail")
+  endif()
 endif()
 
 # some compilers (e.g. NVHPC) have ISO_Fortran_binding.h but don't