More-correct .pc file path fix

- When passing paths to the build system, they are now correctly
  relative to the prefix rather than absolute.

31671.patch

@@ -1,4 +1,4 @@
-From 2bc8a76b14b5db09ceb3d00a081d8411c34451cf Mon Sep 17 00:00:00 2001
+From beb3d7c392c22517c5a91e2f13558bee379abb2a Mon Sep 17 00:00:00 2001
 From: "Benjamin A. Beasley" <code@musicinmybrain.net>
 Date: Wed, 16 Nov 2022 09:00:21 -0500
 Subject: [PATCH] Use CMake variables for paths in pkg-config files
@@ -13,7 +13,7 @@ Use @gRPC_INSTALL_INCLUDEDIR@ for consistency.
  1 file changed, 2 insertions(+), 2 deletions(-)
 
 diff --git a/cmake/pkg-config-template.pc.in b/cmake/pkg-config-template.pc.in
-index d56154bb3a53..3a260418a815 100644
+index d56154bb3a53..5a56b4f88530 100644
 --- a/cmake/pkg-config-template.pc.in
 +++ b/cmake/pkg-config-template.pc.in
 @@ -1,7 +1,7 @@
@@ -21,8 +21,8 @@ index d56154bb3a53..3a260418a815 100644
  exec_prefix=${prefix}
 -includedir=${prefix}/include
 -libdir=${exec_prefix}/lib
-+includedir=@gRPC_INSTALL_INCLUDEDIR@
-+libdir=@gRPC_INSTALL_LIBDIR@
++includedir=${prefix}/@gRPC_INSTALL_INCLUDEDIR@
++libdir=${exec_prefix}/@gRPC_INSTALL_LIBDIR@
  
  Name: @PC_NAME@
  Description: @PC_DESCRIPTION@

grpc.spec

@@ -858,6 +858,12 @@ sed -r -i 's/(std=c\+\+)14/\1%{cpp_std}/g' \
 %build
 # ~~~~ C (core) and C++ (cpp) ~~~~
 
+# Length of the prefix (e.g. /usr), plus a trailing slash (or newline), plus
+# one, to get the index of the first relative path character after the prefix.
+# This is needed because gRPC_INSTALL_*DIR options expect paths relative to the
+# prefix, and supplying absolute paths causes certain subtle problems.
+%global rmprefix %(echo $(($(wc -c <<<'%{_prefix}')+1)))
+
 echo '===== Building C (core) and C++ components =====' 2>&1
 # We could use either make or ninja as the backend; ninja is faster and has no
 # disadvantages (except a small additional BR, given we already need Python)
@@ -867,11 +873,11 @@ echo '===== Building C (core) and C++ components =====' 2>&1
     -DgRPC_INSTALL:BOOL=ON \
     -DCMAKE_CXX_STANDARD:STRING=%{cpp_std} \
     -DCMAKE_SKIP_INSTALL_RPATH:BOOL=ON \
-    -DgRPC_INSTALL_BINDIR:PATH=%{_bindir} \
-    -DgRPC_INSTALL_LIBDIR:PATH=%{_libdir} \
-    -DgRPC_INSTALL_INCLUDEDIR:PATH=%{_includedir} \
-    -DgRPC_INSTALL_CMAKEDIR:PATH=%{_libdir}/cmake/grpc \
-    -DgRPC_INSTALL_SHAREDIR:PATH=%{_datadir}/grpc \
+    -DgRPC_INSTALL_BINDIR:PATH=%(cut -b %{rmprefix}- <<<'%{_bindir}') \
+    -DgRPC_INSTALL_LIBDIR:PATH=%(cut -b %{rmprefix}- <<<'%{_libdir}') \
+    -DgRPC_INSTALL_INCLUDEDIR:PATH=%(cut -b %{rmprefix}- <<<'%{_includedir}') \
+    -DgRPC_INSTALL_CMAKEDIR:PATH=%(cut -b %{rmprefix}- <<<'%{_libdir}/cmake/grpc') \
+    -DgRPC_INSTALL_SHAREDIR:PATH=%(cut -b %{rmprefix}- <<<'%{_datadir}/grpc') \
     -DgRPC_BUILD_TESTS:BOOL=%{?with_core_tests:ON}%{?!with_core_tests:OFF} \
     -DgRPC_BUILD_CODEGEN:BOOL=ON \
     -DgRPC_BUILD_CSHARP_EXT:BOOL=ON \