cmake: Install libsecp256k1.pc file

This change allows downstream projects to use pkg-config to search for
the libsecp256k1 library that is built with CMake.

cmake/GeneratePkgConfigFile.cmake

@@ -0,0 +1,8 @@
+function(generate_pkg_config_file in_file)
+  set(prefix ${CMAKE_INSTALL_PREFIX})
+  set(exec_prefix \${prefix})
+  set(libdir \${exec_prefix}/${CMAKE_INSTALL_LIBDIR})
+  set(includedir \${prefix}/${CMAKE_INSTALL_INCLUDEDIR})
+  set(PACKAGE_VERSION ${PROJECT_VERSION})
+  configure_file(${in_file} ${PROJECT_NAME}.pc @ONLY)
+endfunction()