Remove unused gcc-linux make rules

I don't want to maintain them or make anyone think they should be used.

README.md

@@ -59,7 +59,7 @@ QPDF is known to build and pass its test suite with mingw (latest version tested
 
 # Additional Notes on Build
 
-QPDF's build system, inspired by [abuild](http://www.abuild.org), can optionally use its own built-in rules rather than using libtool and obeying the compiler specified with configure.  This can be enabled by passing `--with-buildrules=buildrules` where buildrules corresponds to one of the `.mk` files (other than `rules.mk`) in the make directory. This should never be necessary on a UNIX system, but may be necessary on a Windows system.  See [README-windows.md](README-windows.md) for details.  There is a `gcc-linux.mk` file enable `gcc-linux` build rules, but it is intended to help test the build system; Linux users should build with the `libtools` rules, which are enabled by default.
+QPDF's build system, inspired by [abuild](http://www.abuild.org), can optionally use its own built-in rules rather than using libtool and obeying the compiler specified with configure.  This can be enabled by passing `--with-buildrules=buildrules` where buildrules corresponds to one of the `.mk` files (other than `rules.mk`) in the make directory. This should never be necessary on a UNIX system, but may be necessary on a Windows system.  See [README-windows.md](README-windows.md) for details.
 
 The QPDF package provides some executables and a software library.  A user manual can be found in the "doc" directory.  The docbook sources to the user manual can be found in the `manual` directory.
 

make/gcc-linux.mk

@@ -1,87 +0,0 @@
-#
-# This file primarily exists for making it possible to test the build
-# system and external library support from Linux.  However, its use is
-# strongly discouraged; use the (default) libtool rules for building
-# on Linux.
-#
-
-# --- Required interface definitions ---
-
-OBJ=o
-LOBJ=o
-
-# Usage: $(call libname,base)
-define libname
-lib$(1).so
-endef
-
-# Usage: $(call binname,base)
-define binname
-$(1)
-endef
-
-# --- Required rule definitions ---
-
-#                       1   2        3
-# Usage: $(call compile,src,includes,xflags)
-define compile
-	$(CXX) $(CPPFLAGS) $(CXXFLAGS) $(3)\
-		$(call depflags,$(basename $(call src_to_obj,$(1)))) \
-		$(foreach I,$(2),-I$(I)) \
-		-c $(1) -o $(call src_to_obj,$(1))
-endef
-
-#                       1   2
-# Usage: $(call c_compile,src,includes)
-define c_compile
-	$(CC) $(CPPFLAGS) $(CFLAGS) \
-		$(call depflags,$(basename $(call c_src_to_obj,$(1)))) \
-		$(foreach I,$(2),-I$(I)) \
-		-c $(1) -o $(call c_src_to_obj,$(1))
-endef
-
-define libcompile
-	$(CXX) $(CPPFLAGS) $(CXXFLAGS) -fpic \
-		$(call depflags,$(basename $(call src_to_lobj,$(1)))) \
-		$(foreach I,$(2),-I$(I)) \
-		-c $(1) -o $(call src_to_lobj,$(1))
-endef
-define c_libcompile
-	$(CC) $(CPPFLAGS) $(CXXFLAGS) -fpic \
-		$(call depflags,$(basename $(call c_src_to_lobj,$(1)))) \
-		$(foreach I,$(2),-I$(I)) \
-		-c $(1) -o $(call c_src_to_lobj,$(1))
-endef
-
-
-#                        1    2
-# Usage: $(call makeslib,objs,library)
-define makeslib
-	$(RM) $2
-	$(AR) cru $(2) $(1)
-	$(RANLIB) $(2)
-endef
-
-#                       1    2       3       4    5       6        7
-# Usage: $(call makelib,objs,library,ldflags,libs,current,revision,age)
-define makelib
-	$(RM) $(2) $(2).*
-	major=$$(( $(5) - $(7) )); \
-	versuffix=$$major.$(7).$(6); \
-	$(CXX) $(CXXFLAGS) -shared -o $(2).$$versuffix $(1) \
-		-Wl,--soname -Wl,`basename $(2)`.$$major \
-		$(3) $(4) && \
-	ln -s `basename $(2)`.$$versuffix $(2) && \
-	ln -s `basename $(2)`.$$versuffix $(2).$$major
-endef
-
-#                       1    2      3       4
-# Usage: $(call makebin,objs,binary,ldflags,libs)
-define makebin
-	$(CXX) $(CXXFLAGS) $(1) -o $(2) $(LDFLAGS) $(3) $(4)
-endef
-
-# Install target
-
-install: all
-	@echo Automated installation is not supported for buildrules=$(BUILDRULES)