BLD: keep using diffpy/gsl 2.1 for conda package.

Add a remark about stock gsl incompatibility.

Notes: Attempt to use the latest stock gsl=2.2.1=0 resulted
produced failed build on Mac OS X.  The failure is because stock
gsl adds libgcc package which installs PREFIX/lib/libstdc++.6.dylib.
The stock boost is however linked with /usr/lib/libstdc++.6.dylib,
which is incompatible with the libgcc version.  It is thus
impossible to link with both stock boost and stock GSL.

Author: pavoljuhas

conda-recipe/meta.yaml

@@ -15,12 +15,19 @@ requirements:
   build:
     - scons
     - boost 1.61.0
+    # Make sure this resolves to gsl from the diffpy channel.
+    # Stock gsl=2.2.1=0 depends on libgcc package which on
+    # Mac OS X shadows C++ system library with its own
+    # incompatible version.
     - gsl 2.1
+    #
     - libobjcryst >=2017.1.1,2017.1.*
 
   run:
     - boost 1.61.0
+    # Make sure this resolves to gsl from the diffpy channel.
     - gsl >=2.1
+    #
     - libobjcryst >=2017.1.1,2017.1.*
 
 test: