contrib: main.mak: force LIBDIR to lib/

CMAKE_INSTALL_LIBDIR can be lib, lib64 or even lib/<multiarch-tuple> on
Debian, but we already split the contribs by multiarch-tuple /
installation directory at the prefix level for contribs to avoid mixups.

Preventing installation in lib64/ ensure every pc files from contribs/
are in the PKG_CONFIG_PATH when configuring.

Refs #26888

contrib/src/main.mak

@@ -448,7 +448,8 @@ RECONF = mkdir -p -- $(PREFIX)/share/aclocal && \
 CMAKEBUILD := env cmake --build
 CMAKE = cmake . -DCMAKE_TOOLCHAIN_FILE=$(abspath toolchain.cmake) \
 		-DCMAKE_INSTALL_PREFIX:STRING=$(PREFIX) \
-		-DBUILD_SHARED_LIBS:BOOL=OFF
+		-DBUILD_SHARED_LIBS:BOOL=OFF \
+		-DCMAKE_INSTALL_LIBDIR:STRING=lib
 ifdef HAVE_WIN32
 CMAKE += -DCMAKE_DEBUG_POSTFIX:STRING=
 endif