Update documentation for Windows ChimeraX build requirements

[goddard@…]

The following bug report has been submitted:
Platform:        Windows-10-10.0.17134
ChimeraX Version: 0.91 (2019-07-30)
Description
ChimeraX on Windows fails to compile bundles/md_crds/gromacs file xdrfile complaining in its configure script that it cannot find /bin/ld.  Our daily build machine has /bin/ld.  I'm not sure what cygwin package provides ld (gcc?).  Our Windows ChimeraX build instructions does not mention this. 

https://www.cgl.ucsf.edu/chimerax/docs/devel/building.html

The needed prerequisite should be added to the build instructions.

All of the rest of ChimeraX builds on this Windows machine except for the md_crds bundle.

goddard@MSI ~/ucsf/chimerax/src/bundles/md_crds
$ make install
make -C gromacs all
make[1]: Entering directory '/home/goddard/ucsf/chimerax/src/bundles/md_crds/gromacs'
# Use separate makefile for xdr so that PREREQ_MAKE can be turned on
make -f Makefile.xdrfile
make[2]: Entering directory '/home/goddard/ucsf/chimerax/src/bundles/md_crds/gromacs'
tar zxf xdrfile-1.1.tar.gz
for p in patch; do \
        if [ -f $p ] ; then \
                (cd xdrfile-1.1b ; patch -p0) < $p ; \
        fi ; \
done
patching file configure
patching file src/trr2xtc.c
cd xdrfile-1.1b ; env env CC='cl /nologo /EHa /GR /GF /MD ' CXX='cl /nologo /EHa /GR /GF /MD /Zc:inline,rvalueCast,strictStrings /wd4251 ' LDFLAGS=" /Ox /W2" ./configure --enable-shared --with-pic
checking build system type... x86_64-unknown-cygwin
checking host system type... x86_64-unknown-cygwin
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking how to create a ustar tar archive... gnutar
checking for C compiler default output file name... conftest.exe
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables... .exe
checking for suffix of object files... obj
checking whether we are using the GNU C compiler... no
checking whether cl /nologo /EHa /GR /GF /MD  accepts -g... no
checking for cl /nologo /EHa /GR /GF /MD  option to accept ISO C89... none needed
checking for style of include used by make... GNU
checking dependency style of cl /nologo /EHa /GR /GF /MD ... none
checking how to run the C preprocessor... cl /nologo /EHa /GR /GF /MD  -E
********************************************************************
* Note: We have not optimized the C compiler flags on your target  *
* yet, but the default CFLAGS=-O3 should be OK in most cases.      *
* You can override this by setting the CFLAGS environment variable.*
*******************************************************************
checking whether cl /nologo /EHa /GR /GF /MD  accepts -O3... no
*******************************************************************
* Sorry, these optimization settings don't seem to work for       *
* your C compiler. Use make CFLAGS=..., or edit the top Makefile. *
*******************************************************************
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... no
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... no
checking whether byte ordering is bigendian... no
checking for int... yes
checking size of int... 4
checking for long int... yes
checking size of long int... 4
checking for long long int... yes
checking size of long long int... 8
checking that size_t can hold pointers... yes
checking floating-point format... configure: WARNING: Unknown floating-point format

checking for a BSD-compatible install... /usr/bin/install -c
checking whether ln -s works... yes
checking whether make sets $(MAKE)... (cached) yes
checking for a sed that does not truncate output... /usr/bin/sed
checking for non-GNU ld... no
configure: error: no acceptable ld found in $PATH
make[2]: *** [Makefile.xdrfile:34: xdrfile-1.1b] Error 1
make[2]: Leaving directory '/home/goddard/ucsf/chimerax/src/bundles/md_crds/gromacs'
make[1]: *** [Makefile:24: xdrfile-1.1b] Error 2
make[1]: Leaving directory '/home/goddard/ucsf/chimerax/src/bundles/md_crds/gromacs'
make: *** [Makefile:10: xdrfile_lib] Error 2


Log:
UCSF ChimeraX version: 0.91 (2019-07-30)  
© 2016-2019 Regents of the University of California. All rights reserved.  
How to cite UCSF ChimeraX  




OpenGL version: 3.3.0 NVIDIA 398.35
OpenGL renderer: GeForce GTX 1070 with Max-Q Design/PCIe/SSE2
OpenGL vendor: NVIDIA Corporation