title: gcc with -isysroot creates include path that starts with equal sign "=" and compile fails
body: <p>I am cross-compiling using the CodeSourcery toolchain for arm (arm-none-linux-gnueabi). I use -isysroot to point at the <code>/usr/include</code> folder on a rootfs folder, but run into trouble when compiling. I have verified that the include folder is accessible.</p>
  
  <p>Below is the gcc call and the output. Some of the output was removed because I don't think it is relevant.</p>
  
  <p>The preprocessor comes up with an include path that is correct except for the "=" character that it starts with, and indicates that the path does not exist. It then fails to find the header files.</p>
  
  <p>How can I make this work?</p>
  
  <pre><code>willem@jacta:~/Projects/button/Debug$ arm-none-linux-gnueabi-gcc -isysroot=/home/willem/Projects/rootfs -nostdinc -I=/usr/include -O0 -g3 -Wall -c -fmessage-length=0 -v -o src/smd/button/button.o ../src/smd/button/button.c -H
  Using built-in specs.
  Target: arm-none-linux-gnueabi
  Configured with: [ ... stuff omitted ... ]
  Thread model: posix
  gcc version 4.4.1 (Sourcery G++ Lite 2010q1-202) 
  COLLECT_GCC_OPTIONS='-isysroot=/home/willem/Projects/rootfs' '-nostdinc' '-I' '=/usr/include' '-O0' '-g3' '-Wall' '-c' '-fmessage-length=0' '-v' '-o' 'src/smd/button/button.o' '-H' '-march=armv5te' '-funwind-tables'
   /home/willem/Tools/CodeSourcery/Sourcery_G++_Lite/bin/../libexec/gcc/arm-none-linux-gnueabi/4.4.1/cc1 -quiet -nostdinc -v -I =/usr/include -iprefix /home/willem/Tools/CodeSourcery/Sourcery_G++_Lite/bin/../lib/gcc/arm-none-linux-gnueabi/4.4.1/ -isysroot /home/willem/Tools/CodeSourcery/Sourcery_G++_Lite/bin/../arm-none-linux-gnueabi/libc -dD -H -isysroot=/home/willem/Projects/rootfs ../src/smd/button/button.c -quiet -dumpbase button.c -march=armv5te -auxbase-strip src/smd/button/button.o -g3 -O0 -Wall -version -fmessage-length=0 -funwind-tables -o /tmp/ccWnd3Xk.s
  ignoring nonexistent directory "=/home/willem/Projects/rootfs/usr/include"
  #include "..." search starts here:
  #include &lt;...&gt; search starts here:
  End of search list.
  GNU C (Sourcery G++ Lite 2010q1-202) version 4.4.1 (arm-none-linux-gnueabi)
      compiled by GNU C version 4.3.2, GMP version 4.3.1, MPFR version 2.4.2.
  GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
  Compiler executable checksum: 250bf78701f747da89e730786c466148
  . ../src/smd/button/button.h
  ../src/smd/button/button.c:2: error: no include path in which to search for pthread.h
  [ ... etc. ... ]
  </code></pre>
  
  <p>Thanks!</p>
  
  <p>W</p>