make sqrt/libm detection work even in case of CPPFLAGS=-Werror

diff --git a/configure.ac b/configure.ac
index 810c506..c5a0319 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -49,10 +49,28 @@ dnl Whether -lm is required for our math functions
 dnl ---------------------------------------------------------------------------
 
 # we need sqrt and pow, but checking for sqrt should be sufficient
-AC_CHECK_FUNC([sqrt],,[
-AC_CHECK_LIB([m],[sqrt],[MATHLIBS="-lm"],AC_MSG_ERROR([
+AC_ARG_VAR([MATHLIBS],[The libraries required for mathematical functions, e.g. -lm])
+if test "x$MATHLIBS" = "x"; then
+       # We must not compile with -Wall -Werror here:
+       # char *sqrt() conflicts with double sin(const double xx) in any case.
+       CFLAGS_save="$CPPFLAGS"
+       CPPFLAGS_save="$CPPFLAGS"
+       CPPFLAGS=""
+       CFLAGS=""
+       AC_CHECK_FUNC([sqrt],[
+               MATHLIBS=""
+       ],[
+               AC_CHECK_LIB([m],[sqrt],[
+                       MATHLIBS="-lm"
+               ],[
+                       AC_MSG_ERROR([
 *** Could not find sqrt() function
-]))])
+])
+               ])
+       ])
+       CFLAGS="$CFLAGS_save"
+       CPPFLAGS="$CPPFLAGS_save"
+fi
 AC_SUBST([MATHLIBS])