Declare a private dependency on libdl.

The new linker requires that the consumers of a library link against
the libraries that that library depends on, which is supported by
Libs.private.

Fixes #16

configure.ac

@@ -106,6 +106,9 @@ fi
 
 AM_CONDITIONAL(HAS_ZNOW, test x$has_znow = xyes)
 
+AC_CHECK_LIB([dl], [dlopen], [DLOPEN_LIBS="-ldl"])
+AC_SUBST([DLOPEN_LIBS])
+
 case $host_os in
     mingw*)
         # visibility flags aren't supported for windows DLLs, and the

epoxy.pc.in

@@ -8,3 +8,4 @@ Description: epoxy GL dispatch Library
 Version: @PACKAGE_VERSION@
 Cflags: -I${includedir}
 Libs: -L${libdir} -lepoxy
+Libs.private: @DLOPEN_LIBS@