Write a deltachat.pc file at build time

This is writes pkg-config/deltachat.pc file in the target directory,
using the PREFIX environment variable at build time.  If this is
undefined at build time /usr/local is used.

deltachat-ffi/deltachat.pc.in

@@ -0,0 +1,11 @@
+prefix={prefix}
+libdir=${{prefix}}/lib
+includedir=${{prefix}}/include
+
+Name: {name}
+Description: {description}
+URL: {url}
+Version: {version}
+Cflags: -I${{includedir}}
+Libs: -L${{libdir}} -ldeltachat
+Libs.private: {libs_priv}