Fix pkg-config File Generation Again Again

Resubmission of #2001. This switches the `sed` invocations to use `-E`,
extended regex syntax, which is better standardized across platforms.
I guess.

Same test plan:

```
make -C lib clean libzstd.pc
cat lib/libzstd.pc

echo # should fail
make -C lib clean libzstd.pc     LIBDIR=/foo
make -C lib clean libzstd.pc INCLUDEDIR=/foo
make -C lib clean libzstd.pc     LIBDIR=/usr/localfoo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/localfoo
make -C lib clean libzstd.pc     LIBDIR=/usr/local/lib     prefix=/foo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/include prefix=/foo

echo # should succeed
make -C lib clean libzstd.pc     LIBDIR=/usr/local/foo
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/foo
make -C lib clean libzstd.pc     LIBDIR=/usr/local/
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local/
make -C lib clean libzstd.pc     LIBDIR=/usr/local
make -C lib clean libzstd.pc INCLUDEDIR=/usr/local
make -C lib clean libzstd.pc     LIBDIR=/tmp/foo prefix=/tmp
make -C lib clean libzstd.pc INCLUDEDIR=/tmp/foo prefix=/tmp
make -C lib clean libzstd.pc     LIBDIR=/tmp/foo prefix=/tmp/foo
make -C lib clean libzstd.pc INCLUDEDIR=/tmp/foo prefix=/tmp/foo

echo # should also succeed
make -C lib clean libzstd.pc prefix=/foo LIBDIR=/foo/bar INCLUDEDIR=/foo/
cat lib/libzstd.pc

mkdir out
cd out
cmake ../build/cmake
make
cat lib/libzstd.pc
```

build/cmake/lib/CMakeLists.txt

@@ -137,11 +137,12 @@ endif ()
 if (UNIX)
     # pkg-config
     set(PREFIX "${CMAKE_INSTALL_PREFIX}")
-    set(LIBDIR "${CMAKE_INSTALL_FULL_LIBDIR}")
+    set(LIBDIR "${CMAKE_INSTALL_LIBDIR}")
+    set(INCLUDEDIR "${CMAKE_INSTALL_INCLUDEDIR}")
     set(VERSION "${zstd_VERSION}")
     add_custom_target(libzstd.pc ALL
             ${CMAKE_COMMAND} -DIN="${LIBRARY_DIR}/libzstd.pc.in" -DOUT="libzstd.pc"
-            -DPREFIX="${PREFIX}" -DVERSION="${VERSION}"
+            -DPREFIX="${PREFIX}" -DLIBDIR="${LIBDIR}" -DINCLUDEDIR="${INCLUDEDIR}" -DVERSION="${VERSION}"
             -P "${CMAKE_CURRENT_SOURCE_DIR}/pkgconfig.cmake"
             COMMENT "Creating pkg-config file")
 

lib/Makefile

@@ -257,11 +257,31 @@ DESTDIR     ?=
 prefix      ?= /usr/local
 PREFIX      ?= $(prefix)
 exec_prefix ?= $(PREFIX)
-libdir      ?= $(exec_prefix)/lib
+EXEC_PREFIX ?= $(exec_prefix)
+libdir      ?= $(EXEC_PREFIX)/lib
 LIBDIR      ?= $(libdir)
 includedir  ?= $(PREFIX)/include
 INCLUDEDIR  ?= $(includedir)
 
+PCLIBDIR ?= $(shell echo "$(LIBDIR)" | sed -n -E -e "s@^$(EXEC_PREFIX)(/|$$)@@p")
+PCINCDIR ?= $(shell echo "$(INCLUDEDIR)" | sed -n -E -e "s@^$(PREFIX)(/|$$)@@p")
+
+ifeq (,$(PCLIBDIR))
+# Additional prefix check is required, since the empty string is technically a
+# valid PCLIBDIR
+ifeq (,$(shell echo "$(LIBDIR)" | sed -n -E -e "\\@^$(EXEC_PREFIX)(/|$$)@ p"))
+$(error configured libdir ($(LIBDIR)) is outside of prefix ($(PREFIX)), can't generate pkg-config file)
+endif
+endif
+
+ifeq (,$(PCINCDIR))
+# Additional prefix check is required, since the empty string is technically a
+# valid PCINCDIR
+ifeq (,$(shell echo "$(INCLUDEDIR)" | sed -n -E -e "\\@^$(PREFIX)(/|$$)@ p"))
+$(error configured includedir ($(INCLUDEDIR)) is outside of exec_prefix ($(EXEC_PREFIX)), can't generate pkg-config file)
+endif
+endif
+
 ifneq (,$(filter $(shell uname),FreeBSD NetBSD DragonFly))
 PKGCONFIGDIR ?= $(PREFIX)/libdata/pkgconfig
 else
@@ -281,9 +301,11 @@ INSTALL_DATA    ?= $(INSTALL) -m 644
 libzstd.pc:
 libzstd.pc: libzstd.pc.in
 	@echo creating pkgconfig
-	$(Q)sed -e 's|@PREFIX@|$(PREFIX)|' \
-             -e 's|@VERSION@|$(VERSION)|' \
-             $< >$@
+	$(Q)@sed -E -e 's|@PREFIX@|$(PREFIX)|' \
+          -e 's|@LIBDIR@|$(PCLIBDIR)|' \
+          -e 's|@INCLUDEDIR@|$(PCINCDIR)|' \
+          -e 's|@VERSION@|$(VERSION)|' \
+          $< >$@
 
 install: install-pc install-static install-shared install-includes
 	@echo zstd static and shared library installed

lib/libzstd.pc.in

@@ -4,8 +4,8 @@
 
 prefix=@PREFIX@
 exec_prefix=${prefix}
-includedir=${prefix}/include
-libdir=${exec_prefix}/lib
+includedir=${prefix}/@INCLUDEDIR@
+libdir=${exec_prefix}/@LIBDIR@
 
 Name: zstd
 Description: fast lossless compression algorithm library