[librsvg/238-build-failure-on-ubuntu-debian] Fix build failure on Ubuntu/Debian

[Tim Lunn <timl src gnome org>]

commit 3f4d143eb171afe9e62690c8965c9eb4b838a0eb
Author: Tim Lunn <tim feathertop org>
Date:   Sat Mar 31 13:55:02 2018 +1100

    Fix build failure on Ubuntu/Debian
    
    This fixes unresolved symbols when linking with --as-needed
    
    Closes #238

 Makefile.am  | 3 ++-
 configure.ac | 5 +++++
 2 files changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/Makefile.am b/Makefile.am
index dcc15e53..94845600 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -172,7 +172,8 @@ librsvg_@RSVG_API_MAJOR_VERSION@_la_LDFLAGS = \
 librsvg_@RSVG_API_MAJOR_VERSION@_la_LIBADD = \
        $(LIBRSVG_LIBS)         \
        $(LIBM)                 \
-       $(RUST_LIB)
+       $(RUST_LIB)             \
+       $(DLOPEN_LIBS)
 
 librsvgincdir = $(includedir)/librsvg-$(RSVG_API_VERSION)/librsvg
 librsvginc_HEADERS =                   \
diff --git a/configure.ac b/configure.ac
index 63b10627..c9666757 100644
--- a/configure.ac
+++ b/configure.ac
@@ -67,6 +67,11 @@ PKG_PROG_PKG_CONFIG
 LT_INIT([win32-dll])
 LT_LIB_M
 
+# Check if -ldl is needed to use dlopen()
+AC_CHECK_FUNC(dlopen, [],
+              [AC_CHECK_LIB(dl, dlopen, [DLOPEN_LIBS=-ldl])])
+AC_SUBST(DLOPEN_LIBS)
+
 dnl ===========================================================================
 
 AC_CHECK_PROG(CARGO, [cargo], [yes], [no])