build: new `--without-qmi' configure option

For those who don't care about the QMI support through libqmi-glib, or if you're
stuck with glib 2.30 (libqmi-glib requires 2.32), this configure switch allows
disabling the QMI support completely.

The logic to detect cdc-wdm ports is still in place, but the QMI probing is
never launched at them. Also, all QMI-related objects won't be compiled.

src/tests/Makefile.am

@@ -22,6 +22,12 @@ test_modem_helpers_LDADD = \
 	$(top_builddir)/src/libmodem-helpers.la \
 	$(MM_LIBS)
 
+if WITH_QMI
+test_modem_helpers_CPPFLAGS += $(QMI_CFLAGS)
+test_modem_helpers_LDADD += $(QMI_LIBS)
+endif
+
+
 test_charsets_SOURCES = \
 	test-charsets.c
 
@@ -38,6 +44,11 @@ test_charsets_LDADD = \
 	$(top_builddir)/src/libmodem-helpers.la \
 	$(MM_LIBS)
 
+if WITH_QMI
+test_charsets_CPPFLAGS += $(QMI_CFLAGS)
+test_charsets_LDADD += $(QMI_LIBS)
+endif
+
 test_qcdm_serial_port_SOURCES = \
 	test-qcdm-serial-port.c
 
@@ -57,6 +68,11 @@ test_qcdm_serial_port_LDADD = \
 	$(top_builddir)/libqcdm/src/libqcdm.la \
 	-lutil
 
+if WITH_QMI
+test_qcdm_serial_port_CPPFLAGS += $(QMI_CFLAGS)
+test_qcdm_serial_port_LDADD += $(QMI_LIBS)
+endif
+
 test_at_serial_port_SOURCES = \
 	test-at-serial-port.c
 
@@ -71,6 +87,11 @@ test_at_serial_port_LDADD = \
 	$(top_builddir)/src/libmodem-helpers.la \
 	-lutil
 
+if WITH_QMI
+test_at_serial_port_CPPFLAGS += $(QMI_CFLAGS)
+test_at_serial_port_LDADD += $(QMI_LIBS)
+endif
+
 test_sms_part_SOURCES = \
 	test-sms-part.c
 
@@ -87,6 +108,11 @@ test_sms_part_LDADD = \
 	$(top_builddir)/src/libmodem-helpers.la \
 	$(MM_LIBS)
 
+if WITH_QMI
+test_sms_part_CPPFLAGS += $(QMI_CFLAGS)
+test_sms_part_LDADD += $(QMI_LIBS)
+endif
+
 if WITH_TESTS
 
 check-local: test-modem-helpers test-charsets test-qcdm-serial-port test-sms-part