Resolve -Wimplicit-int-conversion warnings

Panel.c:414:49: error: implicit conversion loses integer precision: 'int' to 'short' [-Werror,-Wimplicit-int-conversion]
      this->scrollH = MAXIMUM(this->selectedLen - this->w, 0);
                    ~         ~~~~~~~~~~~~~~~~~~^~~~~~~~~
./Macros.h:11:54: note: expanded from macro 'MAXIMUM'
#define MAXIMUM(a, b)                  ((a) > (b) ? (a) : (b))
                                                     ^

IncSet.c:159:38: error: implicit conversion loses integer precision: 'int' to 'char' [-Werror,-Wimplicit-int-conversion]
         mode->buffer[mode->index] = ch;
                                   ~ ^~

Panel.c:456:24: error: implicit conversion loses integer precision: 'int' to 'char' [-Werror,-Wimplicit-int-conversion]
         buffer[len] = ch;
                     ~ ^~
Panel.c:473:22: error: implicit conversion loses integer precision: 'int' to 'char' [-Werror,-Wimplicit-int-conversion]
         buffer[0] = ch;
                   ~ ^~

IncSet.c

@@ -156,7 +156,7 @@ bool IncSet_handleKey(IncSet* this, int ch, Panel* panel, IncMode_GetPanelValue
       doSearch = false;
    } else if (0 < ch && ch < 255 && isprint((unsigned char)ch)) {
       if (mode->index < INCMODE_MAX) {
-         mode->buffer[mode->index] = ch;
+         mode->buffer[mode->index] = (char) ch;
          mode->index++;
          mode->buffer[mode->index] = 0;
          if (mode->isFilter) {

Panel.c

@@ -453,7 +453,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
       }
 
       if (len < 99) {
-         buffer[len] = ch;
+         buffer[len] = (char) ch;
          buffer[len + 1] = '\0';
       }
 
@@ -470,7 +470,7 @@ HandlerResult Panel_selectByTyping(Panel* this, int ch) {
 
          // if current word did not match,
          // retry considering the character the start of a new word.
-         buffer[0] = ch;
+         buffer[0] = (char) ch;
          buffer[1] = '\0';
       }
 

Panel.h

@@ -67,7 +67,7 @@ struct Panel_ {
    int selectedLen;
    void* eventHandlerState;
    int scrollV;
-   short scrollH;
+   int scrollH;
    bool needsRedraw;
    bool wasFocus;
    FunctionBar* currentBar;

configure.ac

@@ -668,7 +668,8 @@ AS_VAR_IF(CACHEVAR,yes,
 AS_VAR_POPDEF([CACHEVAR])dnl
 ])dnl AX_CHECK_COMPILE_FLAGS
 
-AX_CHECK_COMPILE_FLAG([-Wnull-dereference], [AM_CFLAGS="$AM_CFLAGS -Wnull-dereference"], , [-Werror])
+AX_CHECK_COMPILE_FLAG([-Wimplicit-int-conversion], [AM_CFLAGS="$AM_CFLAGS -Wimplicit-int-conversion"], , [-Werror])
+AX_CHECK_COMPILE_FLAG([-Wnull-dereference],        [AM_CFLAGS="$AM_CFLAGS -Wnull-dereference"],        , [-Werror])
 
 AC_ARG_ENABLE([werror],
               [AS_HELP_STRING([--enable-werror],