[librsvg: 1/2] Update MSRV to Rust 1.51 - nalgebra 0.26 requires it

[Federico Mena Quintero <federico src gnome org>]

commit cd649917f10c8e4ebfb795f71c398e986172b9e4
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Apr 29 10:45:59 2021 -0500

    Update MSRV to Rust 1.51 - nalgebra 0.26 requires it
    
    Version 0.26 of nalgebra is where they introduced support for const
    generics; this appeared in Rust 1.51.
    
    Also, the next version of gtk-rs will require Rust 1.51 as well, and
    librsvg will be depending on that soon.

 COMPILING.md | 2 +-
 configure.ac | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/COMPILING.md b/COMPILING.md
index b2024710..31668f51 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -32,7 +32,7 @@ minimum version is listed here; you may use a newer version instead.
 **Compilers:**
 
 * a C compiler and `make` tool; we recommend GNU `make`.
-* rust 1.48 or later
+* rust 1.51 or later
 * cargo
 
 **Mandatory dependencies:**
diff --git a/configure.ac b/configure.ac
index 872e5925..bc12c23d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -105,7 +105,7 @@ AS_IF(test x$RUSTC = xno,
 dnl MSRV - Minimum Supported Rust Version
 dnl If you change this, please update COMPILING.md
 MINIMUM_RUST_MAJOR=1
-MINIMUM_RUST_MINOR=48
+MINIMUM_RUST_MINOR=51
 
 rust_version=`$RUSTC --version`
 version=`echo "$rust_version" | sed -e 's/^rustc //g'`