Have make targets install-capi-lib,install-pkgconfig work without bui…

…lding binary

1. modified target `install-capi-lib` to detect version without requiring the binary being built.
2. added separate install target for library: `install-capi` instead of using `install` for both binary and library
3. added target `install-capi-pkgconfig` that outputs this example `.pc` file if the binary is missing:

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

   Name: wasmer
   Description: The Wasmer library for running WebAssembly
   Version: 2.3.0
   Cflags: -I${prefix}/include
   Libs: -L${prefix}/lib -lwasmer
   ```

Fixes #2541

Makefile

@@ -648,6 +648,8 @@ DESTDIR ?= /usr/local
 
 install: install-wasmer install-capi-headers install-capi-lib install-pkgconfig install-misc
 
+install-capi: install-capi-headers install-capi-lib install-capi-pkgconfig install-misc
+
 install-wasmer:
 	install -Dm755 target/release/wasmer $(DESTDIR)/bin/wasmer
 
@@ -657,7 +659,7 @@ install-capi-headers:
 
 # Currently implemented for linux only. TODO
 install-capi-lib:
-	pkgver=$$(target/release/wasmer --version | cut -d\  -f2) && \
+	pkgver=$$($(CARGO_BINARY) pkgid --manifest-path lib/c-api/Cargo.toml | sed --posix 's/^.*wasmer-c-api:\([0-9.]*\)$\/\1/') && \
 	shortver="$${pkgver%.*}" && \
 	majorver="$${shortver%.*}" && \
 	install -Dm755 target/release/libwasmer.so "$(DESTDIR)/lib/libwasmer.so.$$pkgver" && \
@@ -668,6 +670,10 @@ install-capi-lib:
 install-misc:
 	install -Dm644 LICENSE "$(DESTDIR)"/share/licenses/wasmer/LICENSE
 
+install-capi-pkgconfig:
+	@pkgver=$$($(CARGO_BINARY) pkgid --manifest-path lib/c-api/Cargo.toml | sed --posix 's/^.*wasmer-c-api:\([0-9.]*\)$\/\1/') && \
+	printf "prefix=%s\nincludedir=\044{prefix}/include\nlibdir=\044{prefix}/lib\n\nName: wasmer\nDescription: The Wasmer library for running WebAssembly\nVersion: %s\nCflags: -I\044{prefix}/include\nLibs: -L\044{prefix}/lib -lwasmer\n" "$(DESTDIR)" "$${pkgver}" | install -Dm644 /dev/stdin "$(DESTDIR)"/lib/pkgconfig/wasmer.pc
+
 install-pkgconfig:
 	# Make sure WASMER_INSTALL_PREFIX is set during build
 	unset WASMER_DIR; \