Add instructions for building toolchain

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@1479 42af7a65-404d-4744-a932-0658087f49c3

configs/skp16c26/README.txt

@@ -34,4 +34,33 @@ BUILDING THE R8C/M16C/M32C GNU TOOLCHAIN
 
    This will download the large source packages for the toolchain and build the toolchain.
    The resulting binaries will be under buildroot/build_m32c.
+
+Cygwin GCC BUILD NOTES
+^^^^^^^^^^^^^^^^^^^^^^
+   On Cygwin, the buildroot 'make' command will fail with an error like:
+
+   "...
+      build/genchecksum cc1-dummy > cc1-checksum.c
+      opening cc1-dummy: No such file or directory
+   ..."
+
+   This is caused because on Cygwin, host executables will be generated with the extension .exe
+   and, apparently, the make variable "exeext" is set incorrectly.  A work around after the
+   above occurs is:
+
+      cd toolchain_build_m32c/gcc-4.2.4-initial/gcc	# Go to the directory where error occurred
+      mv cc1-dummy.exe cc1-dummy			# Rename the executable without .exe
+      rm cc1-checksum.c					# Get rid of the bad generated file
+
+   Then resume the buildroot make:
+
+      cd -						# Back to the buildroot make directory
+      make						# Restart the build
+
+   GCC is built twice.  First a initial, "bootstap" GCC is produced in
+   toolchain_build_m32c/gcc-4.2.4-initial, then the final GCC is produced in
+   toolchain_build_m32c/gcc-4.2.4-final.  The above error will occur twice:  Once for
+   the intial GCC build (see above) and once for the final GCC build. For the final GCC
+   build, the workaround is the same except that the directory will be toolchain_build_m32c/gcc-4.2.4-final/gcc.
+
    
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