Updated autoconf to detect Mac OS X and set LDFLAGS accordingly.

The OSX version of `ld` expects `-Wl,--export_dynamic` not
`-Wl,--export-dynamic`.

This helps address bug #16: #16

At least, it helps address the issue.  To build on Mac OS X with
macports you may need the following:

and finally, xnec2c is an X app so be sure to run it under XQuartz.

Big thanks to Rafal Lukawiecki EI6LA for helping troubleshoot to get
xnec2c building on OSX!

configure.ac

@@ -9,6 +9,34 @@ AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIRS([m4 m4-include])
 AM_MAINTAINER_MODE
 
+# AC_CANONICAL_HOST is needed to access the 'host_os' variable
+AC_CANONICAL_HOST
+
+build_linux=no
+build_windows=no
+build_mac=no
+
+# Detect the target system
+case "${host_os}" in
+    linux*)
+        build_linux=yes
+        ;;
+    cygwin*|mingw*)
+        build_windows=yes
+        ;;
+    darwin*)
+        build_mac=yes
+        ;;
+    *)
+        AC_MSG_ERROR(["OS $host_os is not supported"])
+        ;;
+esac
+
+# Pass the conditionals to automake
+AM_CONDITIONAL([LINUX], [test "$build_linux" = "yes"])
+AM_CONDITIONAL([WINDOWS], [test "$build_windows" = "yes"])
+AM_CONDITIONAL([OSX], [test "$build_mac" = "yes"])
+
 # Set CFLAGS if not set in environment
 AC_MSG_CHECKING([whether configure should try to set CFLAGS])
 if test "x${CFLAGS+set}" = xset; then

src/Makefile.am

@@ -1,5 +1,10 @@
 ## Process this file with automake to produce Makefile.in
+
+if OSX
+AM_LDFLAGS = -Wl,-export_dynamic
+else
 AM_LDFLAGS = -Wl,--export-dynamic -Wl,--as-needed
+endif
 
 # Disable warnings for overlength-strings, nothing we can do about that for GTK resources.
 # Maybe some compilers can't handle the length, but gcc and clang take it fine.