how to write a 'configure.ac' for an* optional* support of libguile.

[Pierre Lindenbaum <pierre.lindenbaum@univ-nantes.fr>]

(Cross-posted https://stackoverflow.com/questions/47819563/how-to-configure-ac-with-optional-gnu-guile-library-with-guile )

Hi all,

I'm looking for a simple example of configure.ac for an optional support of libguile

I'm a complete beginner with configure.ac, I'm trying to create a simple program that would only use the GNU guile library if the user invokes:

```
configure --with-guile
```

so the C program would be something like:

```
#include "config.h"
#include <stdio.h>

#ifdef HAVE_GUILE
#include <libguile.h>
#endif

int main(int argc,char** argv) {
#ifdef HAVE_GUILE
     printf("Guile supported\n");
     scm_init_guile();
#else
     printf("Guile not supported\n");
#endif
return 0;
}
``````

GNU guile uses guile-config compile and guile-config link to obtain the path to the include directory and the libraries.

I've not found a tutorial where the paths above are obtained via an external program.

So far my `configure.ac` is

```
AC_INIT(program, 1.0)

AC_PROG_CC
AC_CONFIG_HEADERS(config.h)

with_guile=no

AC_ARG_WITH(guile, [AS_HELP_STRING([--with-guile], [use gnu guile])],[],[with_guile=yes])


if test "x$with_guile" != no
then
     AC_MSG_CHECKING(for Guile)
     guile-config link > /dev/null || {
         echo "configure: cannot find guile-config; is Guile installed?" 1>&2
         exit 1
       }
     CFLAGS="$CFLAGS `guile-config compile`"
     LDFLAGS="$LDFLAGS `guile-config link`"
     AC_DEFINE([HAVE_GUILE],[1],[Guile supported])

     #PKG_CHECK_MODULES([GUILE],[guile-2.0])
     #AC_CHECK_HEADERS([libguile.h], [], [AC_MSG_ERROR([guile include files not found])])
     #AC_CHECK_LIB([guile], [scm_with_guile], [AC_MSG_ERROR([guile library files not found])])
fi


dnl Process Makefile.in to create Makefile

AC_OUTPUT(Makefile)
```

I've removed AC_CHECK_HEADERS and AC_CHECK_LIB because they doesn't work (the files are not found).

Here I'm lost: how can I add the guile paths to CFLAGS and LDFLAGS, how can I generate HAVE_GUILE in config.h

current Makefile.in:

```
CC=@CC@
LD=@CC@
program: program.o
     $(LD) -o $@ $^
.PHONY: clean
clean:
     rm -f program *.o
```

Thanks in advance for your help,

P.