configure: add check and advice for missing ltdl.h

Stop the build at configure time if we can't find ltdl.h in the standard
include paths, and advise installation of libltdl development package.



git-svn-id: https://hamlib.svn.sourceforge.net/svnroot/hamlib/trunk@3060 7ae35d74-ebe9-4afe-98af-79ac388436b8

configure.ac

@@ -203,6 +203,11 @@ mingw* | pw32* | cygwin*)
   ;;
 esac
 
+dnl Check whether we can actually find ltdl.h
+AC_CHECK_HEADER([ltdl.h], [AC_MSG_RESULT([yes])],
+   [ AC_MSG_ERROR([ltdl.h not found.  Please install the libltdl development
+files package which provides /usr/include/ltdl.h (perhaps 'libltdl-dev').]) ])
+
 dnl Check if C99 struct initializers are supported
 AC_MSG_CHECKING(whether C99 struct/array initializers are supported)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[struct{char a;int b;}s[8]={[3]={.b=5}};]])],[AC_MSG_RESULT(yes)],[AC_MSG_ERROR( ""