Fix the broken GCC strlen optimization check

The -O2 flag needs to be used to detect this check in case other
optimization flags are used. This appends the -fno-optimize-strlen even
if other optimization flags are used as it is safer then and GCC 8.0 to
8.2 versions are becoming more and more rare.

Author: petk

cmake/cmake/modules/PHP/CheckBrokenGccStrlenOpt.cmake

@@ -1,8 +1,8 @@
 #[=============================================================================[
-GCC 8.2 shipped with a strlen() optimization bug, so it didn't properly handle
-the 'char val[1]' struct hack. Fixed in GCC 8.3. See https://bugs.php.net/76510
-and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914. If below check is
-successful the -fno-optimize-strlen compiler flag should be used.
+Early GCC 8 versions shipped with a strlen() optimization bug, so it didn't
+properly handle the 'char val[1]' struct hack. Fixed in GCC 8.3. If below check
+is successful the -fno-optimize-strlen compiler flag should be added.
+See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914
 
 Cache variables:
 
@@ -29,6 +29,8 @@ message(CHECK_START "Checking if GCC has broken strlen() optimization")
 
 cmake_push_check_state(RESET)
   set(CMAKE_REQUIRED_QUIET TRUE)
+  # To reproduce the bug, the -O2 flag needs to be used, for example.
+  set(CMAKE_REQUIRED_FLAGS -O2)
 
   check_source_runs(C [[
     #include <stdlib.h>