Provide variant pkg-config file for multi-threaded static lib

Multi-threaded static library require -pthread to correctly link and works.
The pkg-config we provide tho only works with dynamic multi-threaded library
and won't provide the correct libs and cflags values if lib-mt is used.

To handle this, introduce an env variable MT to permit advanced user to
install and generate a correct pkg-config file for lib-mt.

With MT env set on calling make install-pc, libzstd.pc.in generated a
pkg-config file for a multi-threaded static library.

This was notice while migrating from meson to make build system where
meson generates a correct .pc file while make doesn't.

Signed-off-by: Christian Marangi <[email protected]>

Author: Ansuel

lib/Makefile

@@ -63,6 +63,15 @@ CPPFLAGS_DYNLIB  += -DZSTD_MULTITHREAD # dynamic library build defaults to multi
 LDFLAGS_DYNLIB   += -pthread
 CPPFLAGS_STATICLIB +=                  # static library build defaults to single-threaded
 
+# pkg-config library define.
+# For static single-threaded library declare -pthread in Libs.private
+# For static multi-threaded library declare -pthread in Libs and Cflags
+ifneq ($(MT),)
+  PC_MT_LIB := $(LDFLAGS_DYNLIB)
+else
+  PC_LIB := $(LDFLAGS_DYNLIB)
+endif
+
 
 ifeq ($(findstring GCC,$(CCVER)),GCC)
 decompress/zstd_decompress_block.o : CFLAGS+=-fno-tree-vectorize
@@ -316,7 +325,8 @@ libzstd.pc: libzstd.pc.in
 	        -e 's|@INCLUDEDIR@|$(PCINCPREFIX)$(PCINCDIR)|' \
 	        -e 's|@LIBDIR@|$(PCLIBPREFIX)$(PCLIBDIR)|' \
 	        -e 's|@VERSION@|$(VERSION)|' \
-	        -e 's|@LIBS_PRIVATE@|$(LDFLAGS_DYNLIB)|' \
+	        -e 's|@MT_LIBS@|$(PC_MT_LIB)|' \
+	        -e 's|@LIBS_PRIVATE@|$(PC_LIB)|' \
 	        $< >$@
 
 .PHONY: install

lib/README.md

@@ -33,6 +33,10 @@ For convenience, we provide a build target to generate multi and single threaded
 When linking a POSIX program with a multithreaded version of `libzstd`,
 note that it's necessary to invoke the `-pthread` flag during link stage.
 
+The `.pc` generated from `make install` or `make install-pc` always assume a single-threaded static library
+is compiled. If `make lib-mt` is used, to correctly generate a `.pc` for the multi-threaded static library,
+`make install-mt-pc` is needed.
+
 Multithreading capabilities are exposed
 via the [advanced API defined in `lib/zstd.h`](https://github.com/facebook/zstd/blob/v1.4.3/lib/zstd.h#L351).
 

lib/libzstd.pc.in

@@ -11,6 +11,6 @@ Name: zstd
 Description: fast lossless compression algorithm library
 URL: https://facebook.github.io/zstd/
 Version: @VERSION@
-Libs: -L${libdir} -lzstd
+Libs: -L${libdir} -lzstd @MT_LIBS@
 Libs.private: @LIBS_PRIVATE@
-Cflags: -I${includedir}
+Cflags: -I${includedir} @MT_LIBS@