Don't use ac_check_headers with the action to add driver to the list if

checking more than one header. It makes the driver appear several times in
the list of drivers to build. The driver will be built but fails to install
then (reported by D. Dumont). Use a nested way to check for headers instead.

ChangeLog

@@ -28,6 +28,7 @@ v.0.5dev (ongoing development)
  * t6963: Convert font to 8 byte bitmap
  * convert i2500vfd and mdm166a driver to use 8 byte bitmap custom character
  * ula200: convert custom characters to 8 byte bitmap
+ * Build system: Fix trying to install svga driver twice
 
 v0.5.4
  * Update driver includes and LDFLAGS (fixed glk and bayrad binding error)

acinclude.m4

@@ -426,18 +426,23 @@ dnl			else
 			actdrivers=["$actdrivers SureElec"]
 			;;
 		svga)
-			AC_CHECK_HEADERS([vga.h vgagl.h],[
-				AC_CHECK_LIB(vga, main,[
-					LIBSVGA="-lvga -lvgagl"
-					DRIVERS="$DRIVERS svga${SO}"
-					actdrivers=["$actdrivers svga"]
+			AC_CHECK_HEADER([vga.h], [
+				AC_CHECK_HEADER([vgagl.h],[
+					AC_CHECK_LIB(vga, main,[
+						LIBSVGA="-lvga -lvgagl"
+						DRIVERS="$DRIVERS svga${SO}"
+						actdrivers=["$actdrivers svga"]
+					],[
+dnl					else
+						AC_MSG_WARN([The svga driver needs the vga library])
+					])
 				],[
 dnl				else
-					AC_MSG_WARN([The svga driver needs the vga library])
+					AC_MSG_WARN([The svga driver needs vgagl.h])
 				])
 			],[
 dnl			else
-				AC_MSG_WARN([The svga driver needs vga.h and vgagl.h])
+				AC_MSG_WARN([The svga driver needs vga.h])
 			])
 			;;
 		t6963)