Fix build on Slackware 15

The error was:
cc  -o libpcsclite.so.1 libpcsclite.so.1.p/src_error.c.o libpcsclite.so.1.p/src_libredirect.c.o libpcsclite.so.1.p/src_sys_unix.c.o -Wl,--as-needed -Wl,--no-undefined -shared -fPIC -Wl,--start-group -Wl,-soname,libpcsclite.so.1 -Wl,--end-group
/usr/bin/ld: libpcsclite.so.1.p/src_libredirect.c.o: in function `load_lib':
/usr/src/tmp/pcsc-lite-2.2.1/builddir/../src/libredirect.c:139: undefined reference to `dlopen'
/usr/bin/ld: /usr/src/tmp/pcsc-lite-2.2.1/builddir/../src/libredirect.c:142: undefined reference to `dlerror'
/usr/bin/ld: /usr/src/tmp/pcsc-lite-2.2.1/builddir/../src/libredirect.c:148: undefined reference to `dlsym'

Thanks to Georgi D. Sotirov for the bug report
" Problem building pcsc-lite 2.2.1 on Slackware 15 with meson #187 "

Closes: #187

meson.build

@@ -109,6 +109,9 @@ if get_option('libusb')
   features += 'libusb'
 endif
 
+cc = meson.get_compiler('c')
+dl_deps = cc.find_library('dl', required: false)
+
 polkit_dep = dependency('polkit-gobject-1', required : get_option('polkit'))
 if polkit_dep.found()
   conf_data.set('HAVE_POLKIT', true)
@@ -176,6 +179,7 @@ libpcsclite_src = [
   ]
 libpcsclite = library('pcsclite',
   libpcsclite_src,
+  dependencies : dl_deps,
   include_directories : incdir,
   soversion : 1,
   install : true)
@@ -188,6 +192,7 @@ library('pcsclite_fake',
 # libpcscspy library
 library('pcscspy',
   sources : ['src/spy/libpcscspy.c', 'src/sys_unix.c'],
+  dependencies : dl_deps,
   include_directories : incdir,
   soversion : 0,
   install : true)