Minor fix to cmakelists to avoid rerunning command each build (KhronosGroup#5620)

Author: rkevingibson

CMakeLists.txt

@@ -387,7 +387,8 @@ set(SPIRV_SHARED_LIBRARIES "-lSPIRV-Tools-shared")
 
 # Build pkg-config file
 # Use a first-class target so it's regenerated when relevant files are updated.
-add_custom_target(spirv-tools-pkg-config ALL
+add_custom_command(
+        OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/SPIRV-Tools.pc
         COMMAND ${CMAKE_COMMAND}
                       -DCHANGES_FILE=${CMAKE_CURRENT_SOURCE_DIR}/CHANGES
                       -DTEMPLATE_FILE=${CMAKE_CURRENT_SOURCE_DIR}/cmake/SPIRV-Tools.pc.in
@@ -397,8 +398,9 @@ add_custom_target(spirv-tools-pkg-config ALL
                       -DCMAKE_INSTALL_INCLUDEDIR=${CMAKE_INSTALL_INCLUDEDIR}
                       -DSPIRV_LIBRARIES=${SPIRV_LIBRARIES}
                       -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/write_pkg_config.cmake
-        DEPENDS "CHANGES" "cmake/SPIRV-Tools.pc.in" "cmake/write_pkg_config.cmake")
-add_custom_target(spirv-tools-shared-pkg-config ALL
+        DEPENDS "CHANGES" "${CMAKE_CURRENT_SOURCE_DIR}/cmake/SPIRV-Tools.pc.in" "${CMAKE_CURRENT_SOURCE_DIR}/cmake/write_pkg_config.cmake")
+add_custom_command(
+        OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/SPIRV-Tools-shared.pc
         COMMAND ${CMAKE_COMMAND}
                       -DCHANGES_FILE=${CMAKE_CURRENT_SOURCE_DIR}/CHANGES
                       -DTEMPLATE_FILE=${CMAKE_CURRENT_SOURCE_DIR}/cmake/SPIRV-Tools-shared.pc.in
@@ -408,7 +410,10 @@ add_custom_target(spirv-tools-shared-pkg-config ALL
                       -DCMAKE_INSTALL_INCLUDEDIR=${CMAKE_INSTALL_INCLUDEDIR}
                       -DSPIRV_SHARED_LIBRARIES=${SPIRV_SHARED_LIBRARIES}
                       -P ${CMAKE_CURRENT_SOURCE_DIR}/cmake/write_pkg_config.cmake
-        DEPENDS "CHANGES" "cmake/SPIRV-Tools-shared.pc.in" "cmake/write_pkg_config.cmake")
+        DEPENDS "CHANGES" "${CMAKE_CURRENT_SOURCE_DIR}/cmake/SPIRV-Tools-shared.pc.in" "${CMAKE_CURRENT_SOURCE_DIR}/cmake/write_pkg_config.cmake")
+add_custom_target(spirv-tools-pkg-config 
+        ALL 
+        DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/SPIRV-Tools-shared.pc ${CMAKE_CURRENT_BINARY_DIR}/SPIRV-Tools.pc)
 
 # Install pkg-config file
 if (ENABLE_SPIRV_TOOLS_INSTALL)