Add defines for PCI IDs.

This patch allows to set PCI vendor and device IDs using defines
(PCI_VID and PCI_DID).  Use it for vgabios.bin.
diff --git a/Makefile b/Makefile
index c3c744c..d440b93 100644
--- a/Makefile
+++ b/Makefile
@@ -31,8 +31,8 @@
 VBE_FILES := vbe.h vbe.c vbetables.h
 
 # build flags
-vgabios.bin              : VGAFLAGS := -DVBE
-vgabios.debug.bin        : VGAFLAGS := -DVBE -DDEBUG
+vgabios.bin              : VGAFLAGS := -DVBE -DPCI_VID=0x1234
+vgabios.debug.bin        : VGAFLAGS := -DVBE -DPCI_VID=0x1234 -DDEBUG
 vgabios-cirrus.bin       : VGAFLAGS := -DCIRRUS -DPCIBIOS 
 vgabios-cirrus.debug.bin : VGAFLAGS := -DCIRRUS -DPCIBIOS -DCIRRUS_DEBUG
 
diff --git a/vbe.c b/vbe.c
index ecff90d..1fab2f9 100644
--- a/vbe.c
+++ b/vbe.c
@@ -925,7 +925,11 @@
                 if (using_lfb) {
                   info.NumberOfBanks = 1;
                 }
-                lfb_addr = pci_get_lfb_addr(0x1234); // experimental vendor
+#ifdef PCI_VID
+                lfb_addr = pci_get_lfb_addr(PCI_VID);
+#else
+                lfb_addr = 0;
+#endif
                 if (lfb_addr > 0) {
                   info.PhysBasePtr = ((Bit32u)lfb_addr << 16);
                 }
diff --git a/vgabios.c b/vgabios.c
index ea3aae8..2e8b5d7 100644
--- a/vgabios.c
+++ b/vgabios.c
@@ -210,8 +210,13 @@
 .word 0x1013
 .word 0x00b8 // CLGD5446
 #else
+#ifdef PCI_VID
+.word PCI_VID
+.word PCI_DID
+#else
 #error "Unknown PCI vendor and device id"
 #endif
+#endif
 .word 0 // reserved
 .word 0x18 // dlen
 .byte 0 // revision