Introduce LibCore and move GElapsedTimer => CElapsedTimer.

I need a layer somewhere between AK (usable both by userspace and kernel)
and LibGUI (usable by userspace except WindowServer.) So here's LibCore.

LibCore/.gitignore

@@ -0,0 +1,3 @@
+*.o
+*.d
+libcore.a

LibCore/CElapsedTimer.cpp

@@ -0,0 +1,29 @@
+#include <LibCore/CElapsedTimer.h>
+#include <AK/Assertions.h>
+#include <sys/time.h>
+
+void CElapsedTimer::start()
+{
+    m_valid = true;
+    gettimeofday(&m_start_time, nullptr);
+}
+
+inline void timersub(const struct timeval* a, const struct timeval* b, struct timeval* result)
+{
+    result->tv_sec = a->tv_sec - b->tv_sec;
+    result->tv_usec = a->tv_usec - b->tv_usec;
+    if (result->tv_usec < 0) {
+        --result->tv_sec;
+        result->tv_usec += 1000000;
+    }
+}
+
+int CElapsedTimer::elapsed() const
+{
+    ASSERT(is_valid());
+    struct timeval now;
+    gettimeofday(&now, nullptr);
+    struct timeval diff;
+    timersub(&now, &m_start_time, &diff);
+    return diff.tv_sec * 1000 + diff.tv_usec / 1000;
+}

LibCore/CElapsedTimer.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <time.h>
+
+class CElapsedTimer {
+public:
+    CElapsedTimer() { }
+
+    bool is_valid() const { return m_valid; }
+    void start();
+    int elapsed() const;
+
+private:
+    bool m_valid { false };
+    struct timeval m_start_time { 0, 0 };
+};

LibCore/Makefile

@@ -0,0 +1,32 @@
+OBJS = \
+    CElapsedTimer.o
+
+LIBS =
+
+LIBRARY = libcore.a
+STANDARD_FLAGS = -std=c++17 -Wno-sized-deallocation
+WARNING_FLAGS = -Wextra -Wall -Wundef -Wcast-qual -Wwrite-strings -Wimplicit-fallthrough
+FLAVOR_FLAGS = -fno-exceptions -fno-rtti
+OPTIMIZATION_FLAGS = -Os
+INCLUDE_FLAGS = -I../LibC -I../Servers -I.. -I.
+
+DEFINES = -DSERENITY -DUSERLAND -DSANITIZE_PTRS -DLIBGUI
+
+CXXFLAGS = -MMD -MP $(WARNING_FLAGS) $(OPTIMIZATION_FLAGS) $(FLAVOR_FLAGS) $(STANDARD_FLAGS) $(INCLUDE_FLAGS) $(DEFINES)
+CXX = i686-pc-serenity-g++
+LD = i686-pc-serenity-ld
+AR = i686-pc-serenity-ar
+
+all: $(LIBRARY)
+
+$(LIBRARY): $(OBJS)
+	@echo "LIB $@"; $(AR) rcs $@ $(OBJS) $(LIBS)
+
+.cpp.o:
+	@echo "CXX $<"; $(CXX) $(CXXFLAGS) -o $@ -c $<
+
+-include $(OBJS:%.o=%.d)
+
+clean:
+	@echo "CLEAN"; rm -f $(LIBRARY) $(OBJS) *.d
+