libgcc.a vs libgcc_s.so

[Greg Troxel <gdt%lexort.com@localhost>]

I am trying to build geos from git master (it's ok in pkgsrc), and see
the following:

  /bin/sh ../libtool  --tag=CXX    --mode=link g++ -std=c++11 -DGEOS_INLINE  -pedantic -Wall -Wno-long-long  -ffloat-store -DUSE_UNSTABLE_GEOS_CPP_API  -g -O2 -release 3.8.0  -no-undefined -L/usr/pkg/lib -R/usr/pkg/lib -o libgeos.la -rpath /usr/pkg/lib inlines.lo algorithm/libalgorithm.la  geom/libgeom.la  geomgraph/libgeomgraph.la  index/libindex.la  io/libio.la  linearref/liblinearref.la  noding/libnoding.la  operation/liboperation.la  planargraph/libplanargraph.la  precision/libprecision.la  simplify/libsimplify.la  triangulate/libtriangulate.la  util/libutil.la 

  *** Warning: linker path does not have real file for library -lgcc.
  *** I have the capability to make that library automatically link in when
  *** you link to this library.  But I can only do this if you have a
  *** shared version of the library, which you do not appear to have
  *** because I did check the linker path looking for a file starting
  *** with libgcc and none of the candidates passed a file format test
  *** using a regex pattern. Last file checked: /usr/lib/libgcc.a
  *** The inter-library dependencies that have been dropped here will be
  *** automatically added whenever a program is linked with this library
  *** or is declared to -dlopen it.

  *** Since this library must not contain undefined symbols,
  *** because either the platform does not support them or
  *** it was explicitly requested with -no-undefined,
  *** libtool will only create a static version of it.

In /usr/lib:

-r--r--r--  1 root  wheel  166934 May 21 15:41 /usr/lib/libgcc.a
-r--r--r--  1 root  wheel   48954 May 21 15:41 /usr/lib/libgcc_eh.a
lrwxr-xr-x  1 root  wheel      25 May 21 15:41 /usr/lib/libgcc_s.so -> ../../lib/libgcc_s.so.1.0
lrwxr-xr-x  1 root  wheel      25 May 21 15:41 /usr/lib/libgcc_s.so.1 -> ../../lib/libgcc_s.so.1.0
lrwxr-xr-x  1 root  wheel      25 May 21 15:41 /usr/lib/libgcc_s.so.1.0 -> ../../lib/libgcc_s.so.1.0

So I wonder why this is happening, and how libgcc.a relates to libgcc_s.so.
Any clues appreciated.