Autoconf: Fortran testing of C bindings

This patch fixes some issues with testing of C bindings in Fortran.
Specifically, some tests are using a C compiler which may be
unconfigured, causing unexpected errors.

The autoconf script now uses the Fortran compiler to test these
bindings, rather than using the C compiler to test for their existence.
A new macro (AX_FC_CHECK_BIND_C) was added to run these tests.

This achieves the actual goal (test of Fortran binding) on top of the
original goal (availability of C function), while ensuring that the actual
compiler of interest (FC) is used in the test.

Two C-based tests are still present in the script for testing the size
of jmp_buf and sigjmp_buf.  The C compiler is now configured with the
AX_MPI macro, and is only used to determine the size of these structs.

ac/configure.ac

@@ -88,8 +88,9 @@ AC_FC_SRCEXT(f90)
 # - This can cause standard AC_PROG_FC tests to fail if FCFLAGS is configured
 #   with flags from another compiler.
 # - I do not yet know how to resolve this possible issue.
-AX_MPI([],
-  [AC_MSG_ERROR([Could not find MPI launcher.])])
+AX_MPI([], [
+  AC_MSG_ERROR([Could not find MPI launcher.])
+])
 
 
 # Explicitly replace FC and LD with MPI wrappers
@@ -233,13 +234,13 @@ AC_CONFIG_COMMANDS(Makefile.dep, [make depend])
 
 
 # POSIX verification tests
-AC_LANG_PUSH([C])
 
 # These symbols may be defined as macros, making them inaccessible by Fortran.
-# The following exist in BSD and Linux, so we just test for them.
-AC_CHECK_FUNC([setjmp], [], [AC_MSG_ERROR([Could not find setjmp.])])
-AC_CHECK_FUNC([longjmp], [], [AC_MSG_ERROR([Could not find longjmp.])])
-AC_CHECK_FUNC([siglongjmp], [], [AC_MSG_ERROR([Could not find siglongjmp.])])
+# These three exist in modern BSD and Linux libc, so we just confirm them.
+# But one day, we many need to handle them more carefully.
+AX_FC_CHECK_BIND_C([setjmp], [], [AC_MSG_ERROR([Could not find setjmp.])])
+AX_FC_CHECK_BIND_C([longjmp], [], [AC_MSG_ERROR([Could not find longjmp.])])
+AX_FC_CHECK_BIND_C([siglongjmp], [], [AC_MSG_ERROR([Could not find siglongjmp.])])
 
 # Determine the sigsetjmp symbol.  If missing, then point to sigsetjmp_missing.
 #
@@ -248,14 +249,20 @@ AC_CHECK_FUNC([siglongjmp], [], [AC_MSG_ERROR([Could not find siglongjmp.])])
 #   __sigsetjmp     glibc (Linux)
 SIGSETJMP="sigsetjmp_missing"
 for sigsetjmp_fn in sigsetjmp __sigsetjmp; do
-  AC_CHECK_FUNC([${sigsetjmp_fn}], [
+  AX_FC_CHECK_BIND_C([${sigsetjmp_fn}], [
     SIGSETJMP=${sigsetjmp_fn}
     break
   ])
 done
 AC_DEFINE_UNQUOTED([SIGSETJMP_NAME], ["${SIGSETJMP}"])
 
-# Determine the size of jmp_buf and sigjmp_buf
+# Verify the size of nonlocal jump buffer structs
+# NOTE: This requires C compiler, but can it be done with a Fortran compiler?
+AC_LANG_PUSH([C])
+
+AX_MPI([], [AC_MSG_ERROR([Could not find MPI launcher.])])
+AC_SUBST([CC], [$MPICC])
+
 AC_CHECK_SIZEOF([jmp_buf], [], [#include <setjmp.h>])
 AC_CHECK_SIZEOF([sigjmp_buf], [], [#include <setjmp.h>])
 

ac/m4/ax_fc_check_bind_c.m4

@@ -0,0 +1,42 @@
+dnl AX_FC_CHECK_C_LIB(FUNCTION,
+dnl                   [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND],
+dnl                   [OTHER-LDFLAGS], [OTHER-LIBS])
+dnl
+dnl This macro checks if a C binding is available to the compiler.
+dnl
+dnl Equivalently, it checks if the Fortran compiler can see a C function.
+dnl
+dnl Results are cached in `ax_fc_cv_bind_c_FUNCTION`.
+dnl
+AC_DEFUN([AX_FC_CHECK_BIND_C], [
+  AS_VAR_PUSHDEF([ax_fc_Bind_C], [ax_fc_cv_bind_c_$1])
+  m4_ifval([$4],
+    [ax_fc_bind_c_msg_LDFLAGS=" with $4"],
+    [ax_fc_bind_c_msg_LDFLAGS=""]
+  )
+  AC_CACHE_CHECK(
+    [if $FC can bind $1$ax_fc_bind_c_msg_LDFLAGS], [ax_fc_cv_bind_c_$1], [
+      ax_fc_check_bind_c_save_LDFLAGS=$LDFLAGS
+      LDFLAGS="$4 $LDFLAGS"
+      ax_fc_check_bind_c_save_LIBS=$LIBS
+      LIBS="$5 $LIBS"
+      AC_LINK_IFELSE(
+        [AC_LANG_PROGRAM([],[dnl
+dnl begin code block
+        interface
+        subroutine test() bind(c, name="$1")
+        end subroutine test
+        end interface
+        call test])
+dnl end code block
+        ],
+        [AS_VAR_SET([ax_fc_Bind_C], [yes])],
+        [AS_VAR_SET([ax_fc_Bind_C], [no])]
+      )
+      LDFLAGS=$ax_fc_check_bind_c_save_LDFLAGS
+      LIBS=$ax_fc_check_bind_c_save_LIBS
+    ]
+  )
+  AS_VAR_IF([ax_fc_Bind_C], [yes], [$2], [$3])
+  AS_VAR_POPDEF([ax_fc_Bind_C])
+])

ac/m4/ax_fc_check_c_lib.m4

@@ -2,12 +2,12 @@ dnl AX_FC_CHECK_C_LIB(LIBRARY, FUNCTION,
 dnl                   [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND],
 dnl                   [OTHER-LDFLAGS], [OTHER-LIBS])
 dnl
-dnl This macro checks if a C binding is available to the compiler.
-dnl
-dnl Equivalently, it checks if the Fortran compiler can see a C function.
+dnl This macro checks if a C library can be referenced by a Fortran compiler.
 dnl
 dnl Results are cached in `ax_fc_cv_c_lib_LIBRARY_FUNCTION`.
 dnl
+dnl NOTE: Might be possible to rewrite this to use `AX_FC_CHECK_BIND_C`.
+dnl
 AC_DEFUN([AX_FC_CHECK_C_LIB], [
   AS_VAR_PUSHDEF([ax_fc_C_Lib], [ax_fc_cv_c_lib_$1_$2])
   m4_ifval([$5],