Set C standard to c11

This is needed, because dlt_common.c includes `<QtGlobal>` which includes
implicitly `<qtypes.h>` which contains
```
static_assert(sizeof(ptrdiff_t) == sizeof(size_t), "Weird ptrdiff_t and size_t definitions");
...
```
But `static_assert` in C is available only from c11 standard.
Though this is working with clang, but not with gcc compiler, so we set
the standard explicitly to c11.

CMakeLists.txt

@@ -35,6 +35,8 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
+set(CMAKE_C_STANDARD 11)
+
 if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE "Release" CACHE STRING "Choose the type of build, options are: Debug Release RelWithDebInfo MinSizeRel." FORCE)
 endif()

src/src.pro

@@ -6,7 +6,7 @@ QT_VER_MIN = $$member(QT_VERSION, 1)
 
 CONFIG += c++1z
 *-gcc* {
-    QMAKE_CFLAGS += -std=gnu99
+    QMAKE_CFLAGS += -std=c11
     QMAKE_CFLAGS += -Wall
     QMAKE_CFLAGS += -Wextra
     #QMAKE_CXXFLAGS += -pedantic