configure: fix calls to {gdlib,net-snmp}-config

nut directly calls to {gdlib,net-snmp}-config. In Buildroot,
those are not in the PATH, and nut would catch those of the
system (if they are installed), or miss them entirely.

Fix that by using environment variables that will tell where
to look for them.

Note: libusb also uses libusb-config, but only as a fallback
if pkg-config fails. Since we ensure that pkg-config exists,
and libusb is properly installed before we build nut, there
is no need to fix the libusb-config calls, since they won't
be called at all.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN nut-2.6.5.orig/m4/nut_check_libgd.m4 nut-2.6.5/m4/nut_check_libgd.m4
--- nut-2.6.5.orig/m4/nut_check_libgd.m4	2012-07-31 19:38:56.000000000 +0200
+++ nut-2.6.5/m4/nut_check_libgd.m4	2013-11-01 16:24:02.626549810 +0100
@@ -20,7 +20,7 @@
 	LIBS="-lgd -lpng -lz -ljpeg -lfreetype -lm -lXpm -lX11"
 
 	AC_MSG_CHECKING(for gd version via gdlib-config)
-	GD_VERSION=`gdlib-config --version 2>/dev/null`
+	GD_VERSION=`${GDLIB_CONFIG} --version 2>/dev/null`
 	if test "$?" != "0" -o -z "${GD_VERSION}"; then
 		GD_VERSION="none"
 	fi
@@ -34,9 +34,9 @@
 		AC_MSG_WARN([[If gd detection fails, upgrade gd or use --with-gd-includes and --with-gd-libs]])
 		;;
 	*)
-		CFLAGS="`gdlib-config --includes 2>/dev/null`"
-		LDFLAGS="`gdlib-config --ldflags 2>/dev/null`"
-		LIBS="`gdlib-config --libs 2>/dev/null`"
+		CFLAGS="`${GDLIB_CONFIG} --includes 2>/dev/null`"
+		LDFLAGS="`${GDLIB_CONFIG} --ldflags 2>/dev/null`"
+		LIBS="`${GDLIB_CONFIG} --libs 2>/dev/null`"
 		;;
 	esac
 
diff -durN nut-2.6.5.orig/m4/nut_check_libnetsnmp.m4 nut-2.6.5/m4/nut_check_libnetsnmp.m4
--- nut-2.6.5.orig/m4/nut_check_libnetsnmp.m4	2012-07-31 19:38:56.000000000 +0200
+++ nut-2.6.5/m4/nut_check_libnetsnmp.m4	2013-11-01 16:30:07.398282923 +0100
@@ -15,7 +15,7 @@
 
 	dnl See which version of the Net-SNMP library (if any) is installed
 	AC_MSG_CHECKING(for Net-SNMP version via net-snmp-config)
-	SNMP_VERSION=`net-snmp-config --version 2>/dev/null`
+	SNMP_VERSION=`${NET_SNMP_CONFIG} --version 2>/dev/null`
 	if test "$?" != "0" -o -z "${SNMP_VERSION}"; then
 		SNMP_VERSION="none"
 	fi
@@ -33,7 +33,7 @@
 			CFLAGS="${withval}"
 			;;
 		esac
-	], [CFLAGS="`net-snmp-config --base-cflags 2>/dev/null`"])
+	], [CFLAGS="`${NET_SNMP_CONFIG} --base-cflags 2>/dev/null`"])
 	AC_MSG_RESULT([${CFLAGS}])
 
 	AC_MSG_CHECKING(for Net-SNMP libs)
@@ -48,7 +48,7 @@
 			LIBS="${withval}"
 			;;
 		esac
-	], [LIBS="`net-snmp-config --libs 2>/dev/null`"])
+	], [LIBS="`${NET_SNMP_CONFIG} --libs 2>/dev/null`"])
 	AC_MSG_RESULT([${LIBS}])
 
 	dnl Check if the Net-SNMP library is usable