Update Solaris notes in INSTALL.

Mention the possible "make check" failure on Solaris in the
Solaris-specific section of INSTALL. It was already in
section 4.5 but it is better mention it in the OS-specific
section too.

INSTALL

diff --git a/INSTALL b/INSTALL
index cb7f9bf..fece4b7 100644
--- a/INSTALL
+++ b/INSTALL
@@ -122,6 +122,10 @@
     This can be worked around by passing gl_cv_cc_visibility=no
     as an argument to the configure script.
 
+    test_scripts.sh in "make check" may fail if good enough tools are
+    missing from PATH (/usr/xpg4/bin or /usr/xpg6/bin). See sections
+    4.5 and 3.2 for more information.
+
 
 1.2.6. Tru64