Expand use of pkg-config variables.

[eloj]

Change pkg-config generation (liblz4.pc) such that the path variables, not their values, are used in the definitions of Libs and Cflags, and that $prefix is substituted into libdir and includedir iff they start with its value.

To make the changes more clear, here's the the original output (excl. comments):

prefix=/usr/local
libdir=/usr/local/lib
includedir=/usr/local/include

Name: lz4
Description: extremely fast lossless compression algorithm library
URL: http://www.lz4.org/
Version: 1.9.3
Libs: -L/usr/local/lib -llz4
Cflags: -I/usr/local/include

The new output is:

prefix=/usr/local
libdir=${prefix}/lib
includedir=${prefix}/include

Name: lz4
Description: extremely fast lossless compression algorithm library
URL: http://www.lz4.org/
Version: 1.9.3
Libs: -L${libdir} -llz4
Cflags: -I${includedir}

[Cyan4973]

Thanks @eloj ! Great improvement !