Add spirv-headers pkgconfig file

With installed pkgconfig file other projects build processes can
detest availability of the spirv-headers and require some minimum
version of the spirv-headers to be present in build environment.

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6f01ef0..8b94c29 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -124,4 +124,10 @@
         NAMESPACE "${namespace}"
         DESTINATION "${config_install_dir}"
     )
+
+    configure_file(${CMAKE_SOURCE_DIR}/spirv-headers.pc.in ${CMAKE_BINARY_DIR}/spirv-headers.pc @ONLY )
+    install(
+        FILES "${CMAKE_BINARY_DIR}/spirv-headers.pc"
+        DESTINATION ${CMAKE_INSTALL_DATADIR}/pkgconfig
+    )
 endif()
diff --git a/spirv-headers.pc.in b/spirv-headers.pc.in
new file mode 100644
index 0000000..ccc4205
--- /dev/null
+++ b/spirv-headers.pc.in
@@ -0,0 +1,9 @@
+prefix=@prefix@
+includedir=@includedir@
+
+Name: spirv-headers
+Description: Header files from the SPIR-V registry
+Version: @VERSION@
+Requires:
+Libs:
+Cflags: -I${includedir}