README fix for PKG_CONFIG=true

* README: Add note that if you override pkg-config, you need to set
  libffi flags as well. Thanks to Mark Weaver.

diff --git a/README b/README
index f216d7d..bc9f5df 100644 (file)
--- a/README
+++ b/README
@@ -101,10 +101,10 @@ Guile requires the following external packages:
   - pkg-config
 
     Guile's ./configure script uses pkg-config to discover the correct
-    compile and link options for libgc.  If you don't have pkg-config
-    installed, or you have a version of libgc that doesn't provide a
-    .pc file, you can work around this by setting some variables as
-    part of the configure command-line:
+    compile and link options for libgc and libffi.  If you don't have
+    pkg-config installed, or you have a version of libgc that doesn't
+    provide a .pc file, you can work around this by setting some
+    variables as part of the configure command-line:
 
     - PKG_CONFIG=true
 
@@ -112,6 +112,13 @@ Guile requires the following external packages:
 
     - BDW_GC_LIBS=<linker flags for picking up the libgc library>
 
+    Note that because you're bypassing all pkg-config checks, you will
+    also have to specify libffi flags as well:
+
+    - LIBFFI_CFLAGS=<compile flags for picking up libffi headers>
+
+    - LIBFFI_LIBS=<linker flags for picking up the libffi library>
+
 
 Special Instructions For Some Systems =====================================