Updates supporting CPack RPM generation

Enable command line overrides of default settings
* CPACK_GENERATOR
* CPACK_PACKAGE_RELOCATABLE

Enables command line setting of generated RPM version attributes
* CPACK_RPM_PACKAGE_RELEASE
* CPACK_RPM_PACKAGE_ARCHITECTURE

Adds dependency on pkgconfig RPM when not generating relocatable RPM

cmake/spdlogCPack.cmake

-set(CPACK_GENERATOR
-    TGZ
-    ZIP
-    )
+set(CPACK_GENERATOR "TGZ;ZIP" CACHE STRING "Semicolon separated list of generators")
 
 set(CPACK_INCLUDE_TOPLEVEL_DIRECTORY 0)
 set(CPACK_INSTALL_CMAKE_PROJECTS
@@ -22,11 +19,32 @@ set(CPACK_PACKAGE_VERSION ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PR
 if (PROJECT_VERSION_TWEAK)
     set(CPACK_PACKAGE_VERSION ${CPACK_PACKAGE_VERSION}.${PROJECT_VERSION_TWEAK})
 endif ()
-set(CPACK_PACKAGE_RELOCATABLE ON)
+set(CPACK_PACKAGE_RELOCATABLE ON CACHE BOOL "Build relocatable package")
 
 set(CPACK_RPM_PACKAGE_LICENSE "MIT")
-set(CPACK_RPM_PACKAGE_GROUP "System Environment/Libraries")
+set(CPACK_RPM_PACKAGE_GROUP "Development/Libraries")
 set(CPACK_RPM_PACKAGE_URL ${CPACK_PROJECT_URL})
 set(CPACK_RPM_PACKAGE_DESCRIPTION "Very fast, header-only/compiled, C++ logging library.")
 
+if (CPACK_PACKAGE_NAME)
+    set(CPACK_RPM_FILE_NAME "${CPACK_PACKAGE_NAME}-${CPACK_PACKAGE_VERSION}")
+else()
+    set(CPACK_RPM_FILE_NAME "${PROJECT_NAME}-${CPACK_PACKAGE_VERSION}")
+endif()
+
+if (CPACK_RPM_PACKAGE_RELEASE)
+    set(CPACK_RPM_FILE_NAME "${CPACK_RPM_FILE_NAME}-${CPACK_RPM_PACKAGE_RELEASE}")
+endif ()
+
+if (CPACK_RPM_PACKAGE_ARCHITECTURE)
+    set(CPACK_RPM_FILE_NAME "${CPACK_RPM_FILE_NAME}.${CPACK_RPM_PACKAGE_ARCHITECTURE}")
+endif ()
+set(CPACK_RPM_FILE_NAME "${CPACK_RPM_FILE_NAME}.rpm")
+
+if (NOT CPACK_PACKAGE_RELOCATABLE)
+    # Depend on pkgconfig rpm to create the system pkgconfig folder
+    set(CPACK_RPM_PACKAGE_REQUIRES pkgconfig)
+    set(CPACK_RPM_EXCLUDE_FROM_AUTO_FILELIST_ADDITION "${CPACK_PACKAGING_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+endif ()
+
 include(CPack)