the glib configure.ac has:
dnl *** check whether we need preceeding underscores
	AC_CACHE_CHECK([for preceeding underscore in symbols],
		glib_cv_uscore,[
		AC_TRY_RUN([#include <dlfcn.h>
                int glib_underscore_test (void) { return 42; }
		int main() {
		  void *f1 = (void*)0, *f2 = (void*)0, *handle;
		  handle = dlopen ((void*)0, 0);
		  if (handle) {
		    f1 = dlsym (handle, "glib_underscore_test");
		    f2 = dlsym (handle, "_glib_underscore_test");
		  } return (!f2 || f1);
		}],
			[glib_cv_uscore=yes],
			[glib_cv_uscore=no],
			[])
		rm -f plugin.c plugin.$ac_objext plugin.lo
	])
        GLIB_ASSERT_SET(glib_cv_uscore)
	if test "x$glib_cv_uscore" = "xyes"; then
  		G_MODULE_NEED_USCORE=1
	else
  		G_MODULE_NEED_USCORE=0
	fi

in the cross-compile case, this fails.  it's kind of overkill considering how few targets out there actually use a prefix symbol.  it should default to no here.

if you want to be super paranoid, you can query gcc's __USER_LABEL_PREFIX__ define.  something like:
underscore_prefix=$(
    echo "PREFIX=__USER_LABEL_PREFIX__" \
        | ${CPP-${CC-gcc} -E -P} - 2>&1 \
        | ${EGREP-grep} "^PREFIX=" \
        | ${SED-sed} "s:^PREFIX=::"`
)