Add pkg-config file

Makefile.am

@@ -20,6 +20,11 @@ MAINTAINERCLEANFILES = Makefile.in aclocal.m4 ar-lib compile config.guess \
 
 ACLOCAL_AMFLAGS = -I m4
 
+if SHARED_MODULE
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = ngspice.pc
+endif SHARED_MODULE
+
 dist-hook:
 	rm -rf `find $(distdir)/xgraph -name .deps`
 	rm -rf `find $(distdir)/xgraph -name config.status`

configure.ac

@@ -1071,6 +1071,7 @@ fi
 AC_CONFIG_FILES([Makefile
                  man/Makefile
                  man/man1/Makefile
+                 ngspice.pc
                  src/Makefile
                  src/spicelib/Makefile
                  src/spicelib/analysis/Makefile

ngspice.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: libngspice
+Description: General-purpose circuit simulator
+Version: @VERSION@
+Libs: -L${libdir} -lngspice
+Cflags: -I${includedir}
+