Fix trailing whitespace and convert a few tabs to spaces

... according to CMP

documentation/src/basics.dox

@@ -455,20 +455,20 @@ SRCS   = CubeMain.cxx CubeView.cxx CubeViewUI.cxx
 .SUFFIXES: .o .cxx
 
 %.o: %.cxx
-	$(CXX) $(CXXFLAGS) $(DEBUG) -c $<
+        $(CXX) $(CXXFLAGS) $(DEBUG) -c $<
 
 all: $(TARGET)
 
 $(TARGET): $(OBJS)
-	$(LINK) -o $(TARGET) $(OBJS) $(LDSTATIC)
+        $(LINK) -o $(TARGET) $(OBJS) $(LDSTATIC)
 
 CubeMain.o:   CubeMain.cxx CubeViewUI.h
 CubeView.o:   CubeView.cxx CubeView.h CubeViewUI.h
 CubeViewUI.o: CubeViewUI.cxx CubeView.h
 
 clean:
-	rm -f *.o 2> /dev/null
-	rm -f $(TARGET) 2> /dev/null
+        rm -f *.o 2> /dev/null
+        rm -f $(TARGET) 2> /dev/null
 
 \endcode
 

fluid/Project.cxx

@@ -14,7 +14,7 @@
 //     https://www.fltk.org/bugs.php
 //
 
-#include <errno.h>	// strerror(errno)
+#include <errno.h>      // strerror(errno)
 #include "Project.h"
 
 #include "io/String_Writer.h"

fluid/app/templates.cxx

@@ -13,7 +13,7 @@
 //
 //     https://www.fltk.org/bugs.php
 //
-#include <errno.h>		// strerror(errno)
+#include <errno.h>              // strerror(errno)
 #include "app/templates.h"
 
 #include "Fluid.h"

test/checkers.cxx

@@ -635,7 +635,7 @@ const char _usermoves[] =
 // White moves from D3 to E4 (+14).
 // Black's move? G6 H5             <-- try this in GUI mode w/o the fix below
 // Valid moves are:
-// F5 D3	- Black jumps from F5 to D3, E4 removed (-14).
+// F5 D3        - Black jumps from F5 to D3, E4 removed (-14).
 //
 // Hint for further debugging: enable the printf/fflush statements below and
 // set a breakpoint inside the `if` condition below in a debugger. Then take