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

[Pierre Lindenbaum]

(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:

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

Thanks in advance for your help,

P.