freetdm: use m4_ifdef() to check for pkg-config macros and print a warning if…

freetdm: use m4_ifdef() to check for pkg-config macros and print a warning if they are not available

...instead of failing at runtime
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>

libs/freetdm/configure.ac

@@ -24,7 +24,12 @@ AC_PROG_MAKE_SET
 AM_PROG_CC_C_O
 AC_PROG_LIBTOOL
 AC_PROG_INSTALL
-PKG_PROG_PKG_CONFIG
+
+# NOTE: pkg-config is used to detect libisdn
+m4_ifdef([PKG_PROG_PKG_CONFIG],
+	[PKG_PROG_PKG_CONFIG],
+	[AC_MSG_WARN([pkg-config missing (required for libisdn detection)])]
+)
 
 AX_COMPILER_VENDOR
 
@@ -305,7 +310,8 @@ AC_ARG_WITH([libisdn],
 if test "${with_libisdn}" != "no"
 then
 	AC_MSG_RESULT([${as_nl}<<>> ftmod_isdn (libisdn stack)])
-	PKG_CHECK_MODULES([libisdn],
+	m4_ifdef([PKG_CHECK_MODULES],
+		[PKG_CHECK_MODULES([libisdn],
 			[libisdn >= 0.0.1],
 			[AC_MSG_CHECKING([libisdn version])
 			 LIBISDN_VERSION="`${PKG_CONFIG} --modversion libisdn`"
@@ -339,7 +345,9 @@ then
 			],
 			[AC_MSG_ERROR([Need libisdn-0.0.1 or higher])]
 		 )
-	AX_LIB_PCAP
+		 AX_LIB_PCAP],
+		[AC_MSG_WARN([pkg-config missing (required for libisdn detection)])]
+	)
 fi
 AM_CONDITIONAL([HAVE_LIBISDN], [test "${HAVE_LIBISDN}" = "yes"])