add new function to pkg.m4 to help static linking

There is a continued interest in being able to do static
linking of executables.  When using pkg-config to determine
linking requirements, the default behavior is to only
specify libraries required for dynamic linking.

In many cases, it suffices even for static linking to
use

pkg-config --libs package

to obtain the linkage string required for ld.  However, for
libraries that have dependencies on non-standard libraries,
this may not be the case.  The cray PMI library is such a case.

On a Cray CLE 5.X system, for example

pkg-config --libs cray-pmi

returns a string like

-L/opt/cray/pmi/5.0.2-1.0000.9906.117.2.ari/lib64 -lpmi

whereas

pkgconfig --static --libs cray-pmi

returns a string (modified for git commits) like

-L/opt/cray/pmi/5.0.2-1.0000.9906.117.2.ari/lib64
-L/opt/cray/ugni/5.0-1.0000.8563.255.10.ari/lib64
-L/opt/cray/alps/6.0.0-2.0000.8544.102.21.ari/lib64
-L/opt/cray/wlm_detect/1.0-1.0000.48964.8.2.ari/lib64
-L/opt/cray/rca/1.0.0-2.0000.49383.133.60.ari/lib64
-lpmi -lpthread -lalpslli -lpthread -lwlm_detect
-lugni -lpthread -lalpsutil -lpthread -lrca

Unfortunately the pkg.m4 does not contain a function for adding
the --static option to PKG_CHECK_MODULES. This commit defines
a new function, PKG_CHECK_MODULES_STATIC, which in addition to
defining the $1_CFLAGS and $1_LIBS variables defined by PKG_CHECK_MODULES,
also defines a $1_STATIC_LIBS.