configure: mutate pc-file for static-only builds

This effectively reverts c9c4fed39339ffde45123013a117e06d0bb9c488.

The mutation was originally removed because no one knew why it was done,
the spec suggests the fields should be independent of the static or
shared nature, the mutation bears risk of overlinking and no other
project I looked at did such a thing.
This works out well for full shared and full static builds. Mixed
builds with some libraries linked static and others shared were not
considered when removing the mutation and apparently pkg-config did
not consider this case either.

Using a custom static-only build of the desired libraries with a mutated
pc-file has become a common workaround for poor mixed build support of
pkg-config and many buildsystems. It's still very much an imperfect
workaround and doesn't do anything for mixed builds when both library
variants were built. Ideally pkg-config and buildsystems should be
improved to be able to handle this in the general case. Realistically
this won't happen in the foreseeable future though.

At least for the pkg-config side and when using a major (modern) linker
the effect can be emulated without mutating the pc-file by using
pkg-config's `--static` flag and telling the linker to only link
actually used libraries (`--as-needed` for GNU bfd, GNU gold, LLVM lld
and mold; `-z ignore` for illumos' linker).

In between the mutation being removed from libass and now being added
again, FriBidi and FreeType adopted this mutation workaround, FreeType
citing meson doing the same as a motivation. Indeed HarfBuzz's meson
build (but not the autotools build) will also mutate its pc-file.
meson itself started mutating pc files in 2018 (see meson commit
3f7c6cf3d6a204e9665faad3c05bb5049f08ac74). Going further back, FFmpeg
already mutates its pc-files since 2009 (see FFmpeg commit
12701f6e31f465377462c0fc2d5b34477c1c422b).

Reinstate the mutation (now with explaining comment) in libass to help
out affected builders and to foster consistency with other projects
while hoping no one runs into overlinking issues.

2 files changed, 15 insertions, 4 deletions

diff --git a/configure.ac b/configure.ac
index 4c4613c..89b60ec 100644
--- a/configure.ac
+++ b/configure.ac
@@ -301,8 +301,19 @@ AC_SUBST([ASFLAGS], ["$ASFLAGS"])
 AC_SUBST([AS], ["$AS"])
 
 ## Relay package configuration to libass.pc.in
-AC_SUBST([PKG_LIBS_PRIVATE], [${pkg_libs}])
-AC_SUBST([PKG_REQUIRES_PRIVATE], [${pkg_requires}])
+AS_IF([test "x$enable_shared" != xno], [
+    AC_SUBST([PKG_LIBS_PUBLIC], [])
+    AC_SUBST([PKG_REQUIRES_PUBLIC], [])
+    AC_SUBST([PKG_LIBS_PRIVATE], [${pkg_libs}])
+    AC_SUBST([PKG_REQUIRES_PRIVATE], [${pkg_requires}])
+], [
+    # Not mentioned by the pkg-config specification, but
+    # a common workaround for poor support for mixed builds
+    AC_SUBST([PKG_LIBS_PUBLIC], [${pkg_libs}])
+    AC_SUBST([PKG_REQUIRES_PUBLIC], [${pkg_requires}])
+    AC_SUBST([PKG_LIBS_PRIVATE], [])
+    AC_SUBST([PKG_REQUIRES_PRIVATE], [])
+])
 
 ## Setup conditionals for use in Makefiles
 AM_CONDITIONAL([ASM], [test "x$can_asm" = xtrue])
diff --git a/libass.pc.in b/libass.pc.in
index 222cc25..48de494 100644
--- a/libass.pc.in
+++ b/libass.pc.in
@@ -6,8 +6,8 @@ includedir=@includedir@
 Name: libass
 Description: LibASS is an SSA/ASS subtitles rendering library
 Version: @PACKAGE_VERSION@
-Requires:
+Requires: @PKG_REQUIRES_PUBLIC@
 Requires.private: @PKG_REQUIRES_PRIVATE@
-Libs: -L${libdir} -lass
+Libs: -L${libdir} -lass @PKG_LIBS_PUBLIC@
 Libs.private: @PKG_LIBS_PRIVATE@
 Cflags: -I${includedir}