Persist CMake variables used during multiple builds in cache

When a variable created during configuration is used during multiple
builds, and isn't written to a generated header, it should be a cache
variable. If it isn't a cache variable, it will be unavailable during
subsequent builds that perform partial configuration, which in turn has
unintended consequences for those builds.

src/CMakeLists.txt

@@ -64,7 +64,8 @@ if(NOT HAVE_STRPTIME)
 	check_symbol_exists(strptime time.h HAVE_STRPTIME)
 	list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS -D${TIME_H_DEFINITIONS_TRY})
 	if(HAVE_STRPTIME)
-		set(TIME_H_DEFINITIONS ${TIME_H_DEFINITIONS_TRY})
+		# set as cache entry to persist across multiple builds
+		set(TIME_H_DEFINITIONS ${TIME_H_DEFINITIONS_TRY} CACHE INTERNAL "Definitions required for time.h")
 	endif()
 endif()
 check_symbol_exists(timegm time.h HAVE_TIMEGM)
@@ -74,7 +75,8 @@ if(NOT HAVE_TIMEGM)
 	check_symbol_exists(timegm time.h HAVE_TIMEGM)
 	list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS -D${TIME_H_DEFINITIONS_TRY})
 	if(HAVE_TIMEGM)
-		set(TIME_H_DEFINITIONS ${TIME_H_DEFINITIONS_TRY})
+		# set as cache entry to persist across multiple builds
+		set(TIME_H_DEFINITIONS ${TIME_H_DEFINITIONS_TRY} CACHE INTERNAL "Definitions required for time.h")
 	endif()
 endif()
 if(WIN32)
@@ -84,10 +86,12 @@ endif()
 # conversion. If strptime() is absent, sscanf() will be used instead.
 if(HAVE_SETLOCALE AND (HAVE_TIMEGM OR HAVE_MKGMTIME))
 	message(STATUS "Enabling date/time conversion")
-	set(TR31_ENABLE_DATETIME_CONVERSION ON)
+	# set as cache entry to persist across multiple builds
+	set(TR31_ENABLE_DATETIME_CONVERSION ON CACHE INTERNAL "Date/time conversion availability")
 else()
 	message(STATUS "Disabling date/time conversion; some tests may fail")
-	set(TR31_ENABLE_DATETIME_CONVERSION OFF)
+	# set as cache entry to persist across multiple builds
+	set(TR31_ENABLE_DATETIME_CONVERSION OFF CACHE INTERNAL "Date/time conversion availability")
 endif()
 
 include(GNUInstallDirs) # provides CMAKE_INSTALL_* variables and good defaults for install()