Switch Emscripten FPU emulation to mpfr

Matches the arm64 build, and avoids precision issues with the vanilla
double path in fpu_ieee.cpp.

There is a slowdown (Speedometer 4.0 FPU benchmark goes from 7.7 to 2.4),
but it's worth it to avoid quirks that prevent things like the scroll
thumbs in Mac OS 8 from positioning correctly.

Updates mihaip/infinite-mac#6

BasiliskII/src/Unix/_em_build_mpfr.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+set -euo pipefail
+
+mkdir -p /emlibs
+cd /tmp
+
+wget https://gmplib.org/download/gmp/gmp-6.2.1.tar.lz
+tar xf gmp-6.2.1.tar.lz
+cd gmp-6.2.1
+emconfigure ./configure \
+    --disable-assembly \
+    --with-readline=no \
+    --enable-cxx \
+    --host none \
+    --prefix=/emlibs
+make -j6
+make install
+cd ..
+
+wget https://www.mpfr.org/mpfr-current/mpfr-4.1.0.tar.xz
+tar xf mpfr-4.1.0.tar.xz
+cd mpfr-4.1.0
+emconfigure ./configure \
+    --enable-assert=none \
+    --disable-thread-safe \
+    --disable-float128 \
+    --disable-decimal-float \
+    --host none \
+    --with-gmp=/emlibs \
+    --prefix=/emlibs
+make -j6
+make install
+cd ..

BasiliskII/src/Unix/configure.ac

@@ -1872,11 +1872,16 @@ for fpe in $FPE_CORE_TEST_ORDER; do
   ieee)
     case $ac_cv_c_float_format in
     IEEE*)
-      if [[ "$target_cpu" = "arm" -o "$target_cpu" = "aarch64" ]]; then
+      if [[ "$target_cpu" = "arm" -o "$target_cpu" = "aarch64" -o "x$WANT_EMSCRIPTEN" = "xyes" ]]; then
         FPE_CORE="IEEE fpu core (MPFR)"
         DEFINES="$DEFINES -DFPU_MPFR"
         FPUSRCS="$UAE_PATH/fpu/fpu_mpfr.cpp"
 	    LIBS="$LIBS -lgmp -lmpfr"
+        if [[ "x$WANT_EMSCRIPTEN" = "xyes" ]]; then
+          dnl Emscripten version of gmp and mpfr built by _em_build_mpfr.sh
+          CPPFLAGS="$CPPFLAGS -I/emlibs/include"
+          LDFLAGS="$LDFLAGS -L/emlibs/lib"
+        fi
 	  else
         FPE_CORE="IEEE fpu core"
         DEFINES="$DEFINES -DFPU_IEEE"