debug/gdb: restore comments

There are two separate issues with gdb configure usage:
1) inspecting build system libraries while cross-compiling;
2) preferring a shared library over static one.

The first usage issue is described and fixed now.
The second issue was described but the notes were removed
for some reason. This patch restores those notes.

Suggested-by: Alexey Neyman <stilor@att.net>
Signed-off-by: Kirill K. Smirnov <kirill.k.smirnov@gmail.com>

scripts/build/debug/300-gdb.sh

@@ -72,6 +72,12 @@ do_debug_gdb_build() {
         # For gdb-cross this combination of flags forces
         # gdb configure to fall back to default '-lexpat' flag
         # which is acceptable.
+        #
+        # NOTE: DO NOT USE --with-libexpat-prefix (until GDB configure is smarter)!!!
+        # It conflicts with a static build: GDB's configure script will find the shared
+        # version of expat and will attempt to link that, despite the -static flag.
+        # The link will fail, and configure will abort with "expat missing or unusable"
+        # message.
         cross_extra_config+=("--with-expat")
         cross_extra_config+=("--without-libexpat-prefix")
 
@@ -183,6 +189,12 @@ do_debug_gdb_build() {
         #
         # To prevent this '--without-libexpat-prefix' flag must be passed.
         # Thus configure falls back to '-lexpat', which is exactly what we want.
+        #
+        # NOTE: DO NOT USE --with-libexpat-prefix (until GDB configure is smarter)!!!
+        # It conflicts with a static build: GDB's configure script will find the shared
+        # version of expat and will attempt to link that, despite the -static flag.
+        # The link will fail, and configure will abort with "expat missing or unusable"
+        # message.
         native_extra_config+=("--with-expat")
         native_extra_config+=("--without-libexpat-prefix")