ICU-22610 Use Requires.private and Libs.private

To prevent overlinking when publishing shared libraries,
specify internal dependencies using Requires.private and
Libs.private.

Use Libs.private in icu-uc.pc for libicudata.so, and
Requires.private in icu-i18n.pc, icu-io.pc, and icu-lx.pc,
so that ICU internal dependencies will not be bound
into clients.

The resulting *.pc files will produce the following
output for icu-i18n and icu-lx, provided pkgconf has
commit 78d53ea0 (2.2.0 or later) which de-duplicates
multiple -L options:

    commit 78d53ea012dfbaf397bf8e6907efac5b51abac56
    Author: Kai Pastor <[email protected]>
    Date:   Fri Feb 23 15:18:08 2024 +0100

        Revise serials, traversal, flattening

  ./configure --enable-shared ...

    #
    # icu-i18n
    #

    % pkgconf --libs  icu-i18n
    -L/opt/local/lib -licui18n

    % pkgconf --libs --static icu-i18n
    -L/opt/local/lib -licui18n -licuuc -licudata -lpthread -lm

    #
    # icu-lx
    #

    % pkgconf  --libs icu-lx
    -L/opt/local/lib -liculx

    % pkgconf  --libs --static icu-lx
    -L/opt/local/lib -liculx -L/usr/lib/x86_64-linux-gnu -licu-le-hb -lharfbuzz -lm -licuuc -licudata -lpthread -lm

  ./configure --disable-shared --enable-static ...

    #
    # icu-i18n
    #

    % pkgconf --libs  icu-i18n
    -L/opt/local/lib -licui18n -licuuc -licudata -lpthread -lm

    % pkgconf  --libs --static icu-i18n
    -L/opt/local/lib -licui18n -licuuc -licudata -lpthread -lm

    #
    # icu-lx
    #

    % pkgconf  --libs icu-lx
    -L/opt/local/lib -liculx -L/usr/lib/x86_64-linux-gnu -licu-le-hb

    % pkgconf  --libs --static icu-lx
    -L/opt/local/lib -liculx -L/usr/lib/x86_64-linux-gnu -licu-le-hb -lharfbuzz -licuuc -licudata -lpthread -lm

icu4c/source/Makefile.in

@@ -275,24 +275,36 @@ config/icu-uc.pc: config/icu.pc Makefile icudefs.mk
 	@cat config/icu.pc > $@
 	@echo "Description: $(PACKAGE_ICU_DESCRIPTION): Common and Data libraries" >> $@
 	@echo "Name: $(PACKAGE)-uc" >> $@
-	@echo "Libs:" '-L$${libdir}' "${ICULIBS_UC}" "${ICULIBS_DT}" >> $@
-	@echo "Libs.private:" '$${baselibs}' >> $@
+ifeq ($(ENABLE_SHARED),)
+	@echo "Libs:" '-L$${libdir}' "${ICULIBS_UC}" "${ICULIBS_DT}" '$${baselibs}' >> $@
+else
+	@echo "Libs:" '-L$${libdir}' "${ICULIBS_UC}" >> $@
+	@echo "Libs.private:" "${ICULIBS_DT}" '$${baselibs}' >> $@
+endif
 	@echo $@ updated.
 
 config/icu-i18n.pc: config/icu.pc Makefile icudefs.mk
 	@cat config/icu.pc > $@
 	@echo "Description: $(PACKAGE_ICU_DESCRIPTION): Internationalization library" >> $@
 	@echo "Name: $(PACKAGE)-i18n" >> $@
+ifeq ($(ENABLE_SHARED),)
 	@echo "Requires: icu-uc" >> $@
-	@echo "Libs:" "${ICULIBS_I18N}" >> $@
+else
+	@echo "Requires.private: icu-uc" >> $@
+endif
+	@echo "Libs:" '-L$${libdir}' "${ICULIBS_I18N}" >> $@
 	@echo $@ updated.
 
 config/icu-io.pc: config/icu.pc Makefile icudefs.mk
 	@cat config/icu.pc > $@
 	@echo "Description: $(PACKAGE_ICU_DESCRIPTION): Stream and I/O Library" >> $@
 	@echo "Name: $(PACKAGE)-io" >> $@
+ifeq ($(ENABLE_SHARED),)
 	@echo "Requires: icu-i18n" >> $@
-	@echo "Libs:" "${ICULIBS_IO}" >> $@
+else
+	@echo "Requires.private: icu-i18n" >> $@
+endif
+	@echo "Libs:" '-L$${libdir}' "${ICULIBS_IO}" >> $@
 	@echo $@ updated.
 
 ICULEHB_LIBS=@ICULEHB_LIBS@
@@ -306,12 +318,20 @@ config/icu-lx.pc: config/icu.pc Makefile icudefs.mk
 	@cat config/icu.pc > $@
 	@echo "Description: $(PACKAGE_ICU_DESCRIPTION): Paragraph Layout library $(USING_HB)" >> $@
 	@echo "Name: $(PACKAGE)-lx" >> $@
+ifeq ($(ENABLE_SHARED),)
 ifneq ($(ICULEHB_LIBS),)
 	@echo "Requires: icu-le-hb icu-uc" >> $@
 else
 	@echo "Requires: icu-le" >> $@
 endif
-	@echo "Libs:" "${ICULIBS_LX}" >> $@
+else
+ifneq ($(ICULEHB_LIBS),)
+	@echo "Requires.private: icu-le-hb icu-uc" >> $@
+else
+	@echo "Requires.private: icu-le" >> $@
+endif
+endif
+	@echo "Libs:" '-L$${libdir}' "${ICULIBS_LX}" >> $@
 	@echo $@ updated.