Fix detection of -lm in configure

On mingw-w64, libm.a is a dummy library. The existing link test would pass, but pointlessly add -lm to the linking list. Use AC_SEARCH_LIBS instead, so that -lm is only added when strictly needed.
jberdine · May 24, 2023 · 0c81148 · 0c81148
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diff --git a/Changes b/Changes
@@ -748,7 +748,8 @@ OCaml 5.1.0
   (Xavier Leroy, review by David Allsopp, report by Fabrice Le Fessant)
 
 - #12265: Stop adding -lexecinfo to cclibs (leftover debugging code from the
-  multicore project).
+  multicore project). Harden the feature probe for -lm in configure so -lm is
+  only added if strictly necessary.
   (David Allsopp, review by Sébastien Hinderer)
 
 ### Bug fixes:

diff --git a/configure b/configure
diff --git a/configure.ac b/configure.ac
@@ -966,9 +966,11 @@ AC_PROG_INSTALL
 # Checks for libraries
 
 ## Mathematical library
-AC_CHECK_LIB([m],[cos])
-
-AS_IF([test "x$ac_cv_lib_m_cos" = xyes ], [mathlib="-lm"], [mathlib=""])
+AC_SEARCH_LIBS([cos], [m],
+  [AS_CASE([$ac_cv_search_cos],
+    [no], [AC_MSG_ERROR([Could not locate C math functions])],
+    ['none required'], [mathlib=""],
+    [mathlib="$ac_cv_search_cos"])])
 
 # Checks for header files