Add pcb2gcode_LDFLAGS_EXTRA

This makes it easier to to profiling.  Just run configure like this:

./configure pcb2gcode_CPPFLAGS_EXTRA="-Werror -pg" pcb2gcode_LDFLAGS_EXTRA=-pg --enable-code-coverage --disable-silent-rules --enable-static --disable-shared

Makefile.am

@@ -70,7 +70,7 @@ GERBV_VERSION = `pkg-config --modversion libgerbv`
 
 AM_CPPFLAGS = $(BOOST_CPPFLAGS_SYSTEM) $(glibmm_CFLAGS) $(gdkmm_CFLAGS_SYSTEM) $(gerbv_CFLAGS_SYSTEM) $(CODE_COVERAGE_CPPFLAGS) -DGIT_VERSION=\"$(GIT_VERSION)\" -Wall -Wpedantic -Wextra $(pcb2gcode_CPPFLAGS_EXTRA) $(GEOS_CFLAGS) $(GEOS_EXTRA)
 AM_CXXFLAGS = $(CODE_COVERAGE_CXXFLAGS) -DGIT_VERSION=\"$(GIT_VERSION)\" -DGERBV_VERSION=\"$(GERBV_VERSION)\"
-AM_LDFLAGS = $(BOOST_PROGRAM_OPTIONS_LDFLAGS)
+AM_LDFLAGS = $(BOOST_PROGRAM_OPTIONS_LDFLAGS) $(pcb2gcode_LDFLAGS_EXTRA)
 LIBS = $(gerbv_LIBS) $(BOOST_PROGRAM_OPTIONS_LIBS) $(CODE_COVERAGE_LIBS) $(GEOS_CC_LIBS)
 
 EXTRA_DIST = millproject

configure.ac

@@ -71,6 +71,7 @@ AC_SUBST([gdkmm_CFLAGS_SYSTEM], ['$(subst -I,-isystem ,$(gdkmm_CFLAGS))'])
 AC_SUBST(gerbv_LIBS)
 AC_SUBST(gerbv_CFLAGS)
 AC_ARG_VAR([pcb2gcode_CPPFLAGS_EXTRA], [Additional flags when compiling pcb2gcode])
+AC_ARG_VAR([pcb2gcode_LDFLAGS_EXTRA], [Additional flags when linking pcb2gcode])
 
 # GERBV_INTERPOLATION_LINEARx10 isn't available in older gerbv.h's, but
 # we want to use the new symbol