Add stricter compiler warnings to build system.

These are for standard compliance and should on by default:

    -Wall -Wextra -pedantic

The problem is that even `-Wall` and `-Wextra` does not cover all
warnings, as to not break backward compatibility. Clang therefore
has the `-Weverything` flag, that really includes everything but is
overkill for the day to day development.

Thus, we in addition add:

    -Wuninitialized -Wunreachable-code

to guard against undefined behavior from reading uninitialized variables
and warn for unreachable code.

With:

    -Wstrict-overflow=1

the compiler warns us when it's doing optimizations based on the fact
that signed integer overflows are undefined behavior.

With:

    -D_FORTIFY_SOURCE=2

we tell the compiler to replace functions like strcpy with strncpy where
it can do so, resulting in cheap and useful buffer overflow protection.

References:

- https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
- https://securityblog.redhat.com/2014/03/26/fortify-and-you/
- https://wiki.debian.org/Hardening

CMakeLists.txt

@@ -155,7 +155,7 @@ endif()
 if(${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
   # using Clang
   #  -Weverything -Wno-c++98-compat -Wno-shadow -Wno-exit-time-destructors
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wunreachable-code -pedantic -fPIC")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -pedantic -Wuninitialized -Wunreachable-code -Wstrict-overflow=3 -D_FORTIFY_SOURCE=2 -fPIC")
 elseif(${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
   set(COLOR_FLAG "-fdiagnostics-color=auto")
   check_cxx_compiler_flag("-fdiagnostics-color=auto" HAS_COLOR_FLAG)
@@ -163,7 +163,7 @@ elseif(${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
     set(COLOR_FLAG "")
   endif()
   # using GCC
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -fPIC ${COLOR_FLAG}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -pedantic -Wuninitialized -Wunreachable-code -Wstrict-overflow=3 -D_FORTIFY_SOURCE=2 -fPIC ${COLOR_FLAG}")
   if(WIN32) # using mingw
     add_definitions(-D_USE_MATH_DEFINES) # define M_PI, M_1_PI etc.
     add_definitions(-DWIN32)