defined but not used variables in headers

[schwehr]

Describe the bug

Variables defined with static in headers get put into compilation units where they aren't used.

To Reproduce

On debian testing for example:

gcc --version
gcc (Debian 7.3.0-18) 7.3.0
CFLAGS="-O2 -Wall -Wextra -Wshadow" ./configure --enable-test

gcc -DHAVE_CONFIG_H -I. -I../../src/mbio  -I../../src/mbio -I../../src/mbaux -I../../src/gsf  -I/usr/include/gmt -I/usr/include     -O2 -Wall -Wextra -Wshadow -MT mbinfo.o -MD -MP -MF $depbase.Tpo -c -o mbinfo.o mbinfo.c &&\
mv -f $depbase.Tpo $depbase.Po

[SNIP]

In file included from mbinfo.c:37:0:
At top level:
../../src/mbio/mb_io.h:147:14: warning: ‘mb_sensor_type_string’ defined but not used [-Wunused-variable]
 static char *mb_sensor_type_string[] = {"Unknown sensor type",
              ^~~~~~~~~~~~~~~~~~~~~
../../src/mbio/mb_io.h:123:12: warning: ‘mb_sensor_type_id’ defined but not used [-Wunused-variable]
 static int mb_sensor_type_id[] = {
            ^~~~~~~~~~~~~~~~~
../../src/mbio/mb_io.h:52:14: warning: ‘mb_platform_type_string’ defined but not used [-Wunused-variable]
 static char *mb_platform_type_string[] = {
              ^~~~~~~~~~~~~~~~~~~~~~~