Build: Switch to CMake :^)

Closes https://github.com/SerenityOS/serenity/issues/2080

Libraries/LibMarkdown/CMakeLists.txt

@@ -0,0 +1,11 @@
+set(SOURCES
+    CodeBlock.cpp
+    Document.cpp
+    Heading.cpp
+    List.cpp
+    Paragraph.cpp
+    Text.cpp
+)
+
+serenity_lib(LibMarkdown markdown)
+target_link_libraries(LibMarkdown LibC)

Libraries/LibMarkdown/Makefile

@@ -1,16 +0,0 @@
-OBJS = \
-    Document.o \
-    Paragraph.o \
-    Heading.o \
-    CodeBlock.o \
-    List.o \
-    Text.o
-
-LIBRARY = libmarkdown.a
-
-install:
-	mkdir -p $(SERENITY_BASE_DIR)/Root/usr/include/LibMarkdown/
-	cp *.h $(SERENITY_BASE_DIR)/Root/usr/include/LibMarkdown/
-	cp $(LIBRARY) $(SERENITY_BASE_DIR)/Root/usr/lib/
-
-include ../../Makefile.common