Build fdopen(), hypot(), putenv() in libc.

These functions are in POSIX; hypot() is in C99.  Also build cabs()
because it rides with hypot(), but don't declare cabs() in any header
file, because our compiler can't parse C99 "double complex" type.

Touch build.lua so it sees that .c files moved.

lang/cem/libcc.ansi/build.lua

@@ -37,7 +37,7 @@ for _, plat in ipairs(vars.plats) do
 			"./errno/*.c",
 			"./locale/*.c",
 			"./malloc/*.c",
-			"./math/*.c",
+			"./math/*.c", -- hypot.c
 			"./math/*.e",
 			"./misc/environ.c", -- don't build everything here as it's all obsolete
 			"./setjmp/*.c",

lang/cem/libcc.ansi/headers/math.h

@@ -33,6 +33,7 @@ extern double ceil(double _x);
 extern double fabs(double _x);
 extern double floor(double _x);
 
+extern double hypot(double _x, double _y);
 extern double pow(double _x, double _y);
 
 extern double frexp(double _x, int *_exp);