Keep GNU fastpath for jerry_port_get_local_time_zone_adjustment (#4600)

PR #4513 caused a measurable slowdown by removing the GNU specific TZA calculation.
This patch reverts this removal but also keeps the general implementation introduced in #4513

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik robert.fancsik@h-lab.eu

jerry-port/default/CMakeLists.txt

@@ -59,6 +59,22 @@ endif()
 # (should only be necessary if we used compiler default libc but not checking that)
 set(DEFINES_PORT_DEFAULT _BSD_SOURCE _DEFAULT_SOURCE)
 
+INCLUDE (CheckStructHasMember)
+# CHECK_STRUCT_HAS_MEMBER works by trying to compile some C code that accesses the
+# given field of the given struct. However, our default compiler options break this
+# C code, so turn a couple of them off for this.
+if(USING_GCC OR USING_CLANG)
+  set(CMAKE_REQUIRED_FLAGS "-Wno-error=strict-prototypes -Wno-error=old-style-definition -Wno-error=unused-value")
+endif()
+# tm.tm_gmtoff is non-standard, so glibc doesn't expose it in c99 mode
+# (our default). Define some macros to expose it anyway.
+set(CMAKE_REQUIRED_DEFINITIONS "-D_BSD_SOURCE -D_DEFAULT_SOURCE")
+CHECK_STRUCT_HAS_MEMBER ("struct tm" tm_gmtoff time.h HAVE_TM_GMTOFF)
+# localtime_r is is not threadsafe with clang on OSX
+if(HAVE_TM_GMTOFF AND NOT "${PLATFORM}" STREQUAL "DARWIN" AND NOT USING_CLANG)
+  set(DEFINES_PORT_DEFAULT ${DEFINES_PORT_DEFAULT} HAVE_TM_GMTOFF)
+endif()
+
 # Default Jerry port implementation library
 add_library(${JERRY_PORT_DEFAULT_NAME} ${SOURCE_PORT_DEFAULT})
 target_include_directories(${JERRY_PORT_DEFAULT_NAME} PUBLIC ${INCLUDE_PORT_DEFAULT})