Various makefile fixes to allow cross compilation

Partly ported from
http://anonscm.debian.org/cgit/pkg-games/libsoil.git/tree/debian/patches/linking_correctly.patch

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr "soil.org/projects/makefile/alternate Makefile.txt" "soil/projects/makefile/alternate Makefile.txt"
--- "soil.org/projects/makefile/alternate Makefile.txt"	2008-07-07 18:13:28.000000000 +0200
+++ "soil/projects/makefile/alternate Makefile.txt"	2015-11-07 11:15:04.140106336 +0100
@@ -1,8 +1,8 @@
 MAKE = make
-CC = gcc
-INSTALL_FILE = install -p -o root -g root -m 644
-INSTALL_DIR = install -p -o root -g root -d
-LN = ln -s
+CC = $(GCC)
+INSTALL_FILE = $(INSTALL) -m 644
+INSTALL_DIR = $(INSTALL) -d
+LN = ln -sf
 RM = rm -fv
 CFLAGS += -c -O2 -Wall
 LDFLAGS +=
@@ -23,13 +23,13 @@
 all: $(OFILES) lib
 
 %.o: %.c
-	$(CC) $(CFLAGS) $< -o $@
+	$(CC) $(CFLAGS) -c -fPIC $< -o $@
 
 lib: $(OFILES)
 	# create static library
-	ar -cvq $(LIBNAME).a $(OFILES)
+	$(AR) -cvq $(LIBNAME).a $(OFILES)
 	# create shared library
-	gcc -shared -Wl,-soname,$(LIBNAME).so.$(MAJOR) -o $(LIBNAME).so.$(VERSION) $(OFILES)
+	$(CC) -shared -Wl,-soname,$(LIBNAME).so.$(MAJOR) -o $(LIBNAME).so.$(VERSION) $(OFILES) -lGL -lm
 
 install:
 	$(INSTALL_DIR) $(DESTDIR)/$(INCLUDEDIR)