diff --git a/Makefile.in b/Makefile.in
index 0a81ef0..06b9a28 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -8,7 +8,10 @@
 VERSION=7.3.0w
 FLORIST_VERSION=$(VERSION)
 GNATPREPFLAGS = -c -r
-GCCFLAGS = -O2
+ADAFLAGS = @ADAFLAGS@
+CFLAGS = @CFLAGS@
+CPPFLAGS = @CPPFLAGS@
+LDFLAGS = @LDFLAGS@
 TARGET=@host_alias@
 
 ifneq ($(TARGET),)
@@ -32,13 +35,17 @@ PREFIX = @prefix@
 ENABLE_SHARED = @ENABLE_SHARED@
 
 PROJECT_FLAGS = @BUILD_TYPE_OPTION@ @THREADS_OPTION@ @RTS_OPTION@ \
+                $(foreach v,ADAFLAGS CFLAGS CPPFLAGS LDFLAGS LIBS\
+                           ,'-X$(v)=$($(v))') \
                 $(TARGET_OPTION)
 # Scenario variables and RTS selection must be passed consistently to
 # project-aware tools gprbuild, gprinstall, and gprclean.
+# *FLAGS usually only influence gprbuild, but stay on the safe side.
+# For example gprinstall may copy LIBS into the installed project.
 
 GNATPREP = $(TARGET_PREFIX)gnatprep
 GPRBUILD = gprbuild
-GPRBUILD_FLAGS = $(GCCFLAGS) $(PROJECT_FLAGS)
+GPRBUILD_FLAGS = $(PROJECT_FLAGS)
 
 GENDIR = gensrc
 GENDIR_ON_TARGET=$(GENDIR)
@@ -108,12 +115,12 @@ configure: configure.in aclocal.m4
 # If you are unlucky, it will require fixes to c-posix.c.
 #
 c-posix: c-posix.c confsrc/config.h confsrc/pconfig.h
-	$(CC) $(GCCFLAGS) -DVERSION="\"$(VERSION)\"" -DLIBS="\"$(LIBS)\"" -DGENDIR="\"$(GENDIR_ON_TARGET)\"" -o c-posix c-posix.c $(LIBS)
+	$(CC) -O2 $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -DVERSION="\"$(VERSION)\"" -DLIBS="\"$(LIBS)\"" -DGENDIR="\"$(GENDIR_ON_TARGET)\"" -o c-posix c-posix.c $(LIBS)
 #
 # Program c-posix-signals generates another Ada package spec.
 #
 c-posix-signals: c-posix-signals.c
-	$(CC) $(GCCFLAGS) -DGENDIR="\"$(GENDIR_ON_TARGET)\"" -o c-posix-signals c-posix-signals.c $(LIBS)
+	$(CC) -O2 $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -DGENDIR="\"$(GENDIR_ON_TARGET)\"" -o c-posix-signals c-posix-signals.c $(LIBS)
 #
 # generate Ada source files using "c-posix" program
 #
@@ -140,7 +147,7 @@ $(GENDIR)/%.ads: libsrc/%.gps gnatprep.config
 $(GENDIR)/%.adb: libsrc/%.gpb gnatprep.config
 	mkdir -p `dirname $@` && $(GNATPREP) $< $@ gnatprep.config $(GNATPREPFLAGS)
 .c.o:
-	$(CC) -c $(GCCFLAGS) $<
+	$(CC) -c -O2 $(CFLAGS) $(CPPFLAGS) $<
 
 #
 # ------------------------------------
diff --git a/configure.in b/configure.in
index a5d8040..53d469e 100644
--- a/configure.in
+++ b/configure.in
@@ -13,6 +13,8 @@ AC_PROG_CC
 
 rm -f pconfig.h
 
+AC_ARG_VAR(ADAFLAGS,[Debugging and optimization options for the Ada compiler.])
+
 AC_ARG_ENABLE(threads,
 AS_HELP_STRING([--disable-threads], [Do not try to build pthread support]),,
 [use_pthread=yes])
diff --git a/florist.gpr b/florist.gpr
index faa6ca4..1bad7f0 100644
--- a/florist.gpr
+++ b/florist.gpr
@@ -15,16 +15,24 @@ library project Florist is
           for Source_Dirs use Common_Source_Dirs & Threads_Source_Dirs;
    end case;
 
-   for Object_Dir   use "obj";
-   for Library_Dir  use "lib";
-
-   Version := "1";
+   Version := External ("florist_shared_object_version", "1");
    for Library_version use "libflorist.so." & Version;
 
    type Library_Type_Type is ("relocatable", "static");
    Library_Type : Library_Type_Type := external ("LIBRARY_TYPE", "static");
    for Library_Kind use Library_Type;
 
+   for Object_Dir  use "obj/" & Library_Type;
+   for Library_Dir use "lib/" & Library_Type;
+
+   case Library_Type is
+      when "relocatable" =>
+         for Leading_Library_Options use External_As_List ("LDFLAGS", " ");
+         for Library_Options use External_As_List ("LIBS", " ");
+      when "static" =>
+         null;
+   end case;
+
    type Build_Type is ("Debug", "Production");
    Build : Build_Type := External ("Build", "Production");
 
@@ -41,12 +49,20 @@ library project Florist is
             GNAT_Flags := ("-g", "-O2", "-gnatpg");
       end case;
 
+      --  Always let ADAFLAGS take priority.
+      Adaflags := External_As_List ("ADAFLAGS", " ");
+      Ada_Flags  := Ada_Flags  & Adaflags;
+      GNAT_Flags := GNAT_Flags & Adaflags;
+
       for Switches ("posix-signals.adb") use GNAT_Flags;
       for Switches ("posix-implementation.adb") use GNAT_Flags;
       for Switches ("posix-supplement_to_ada_io.adb") use GNAT_Flags;
       for Switches ("posix-unsafe_process_primitives.adb") use GNAT_Flags;
       for Default_Switches ("Ada") use Ada_Flags;
 
+      for Default_Switches ("C") use ("-O2")
+        & External_As_List ("CFLAGS", " ")
+        & External_As_List ("CPPFLAGS", " ");
    end Compiler;
 
 end Florist;