write_basic_package_version_file: use PROJECT_VERSION

In the write_basic_package_version_file(), the VERSION argument
is now optional. If none is given, it falls back to ${PROJECT_VERSION}.

Alex

Modules/CMakePackageConfigHelpers.cmake

@@ -99,7 +99,7 @@
 #
 # ::
 #
-#   WRITE_BASIC_PACKAGE_VERSION_FILE( filename VERSION major.minor.patch COMPATIBILITY (AnyNewerVersion|SameMajorVersion|ExactVersion) )
+#   WRITE_BASIC_PACKAGE_VERSION_FILE( filename [VERSION major.minor.patch] COMPATIBILITY (AnyNewerVersion|SameMajorVersion|ExactVersion) )
 #
 #
 #
@@ -112,6 +112,9 @@
 #     filename is the output filename, it should be in the build tree.
 #     major.minor.patch is the version number of the project to be installed
 #
+# If no ``VERSION`` is given, the :variable:`PROJECT_VERSION` variable
+# is used.  If this hasn't been set, it errors out.
+#
 # The COMPATIBILITY mode AnyNewerVersion means that the installed
 # package version will be considered compatible if it is newer or
 # exactly the same as the requested version.  This mode should be used

Modules/WriteBasicConfigVersionFile.cmake

@@ -6,7 +6,7 @@
 #
 # ::
 #
-#   WRITE_BASIC_CONFIG_VERSION_FILE( filename VERSION major.minor.patch COMPATIBILITY (AnyNewerVersion|SameMajorVersion) )
+#   WRITE_BASIC_CONFIG_VERSION_FILE( filename [VERSION major.minor.patch] COMPATIBILITY (AnyNewerVersion|SameMajorVersion) )
 #
 #
 #
@@ -46,7 +46,11 @@ function(WRITE_BASIC_CONFIG_VERSION_FILE _filename)
   endif()
 
   if("${CVF_VERSION}" STREQUAL "")
-    message(FATAL_ERROR "No VERSION specified for WRITE_BASIC_CONFIG_VERSION_FILE()")
+    if ("${PROJECT_VERSION}" STREQUAL "")
+      message(FATAL_ERROR "No VERSION specified for WRITE_BASIC_CONFIG_VERSION_FILE()")
+    else()
+      set(CVF_VERSION "${PROJECT_VERSION}")
+    endif()
   endif()
 
   configure_file("${versionTemplateFile}" "${_filename}" @ONLY)